High mass loading of h-WO3 and α-MnO2 on flexible carbon cloth for high-energy aqueous asymmetric supercapacitor

Electrochimica Acta 299 (2019) 245e252

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Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta

High mass loading of h-WO3 and a-MnO2 on flexible carbon cloth for high-energy aqueous asymmetric supercapacitor Su-Hyeon Ji, Nilesh R. Chodankar, Woo-Sung Jang, Do-Heyoung Kim* School of Chemical Engineering, Chonnam National University, Gwangju, 61186, South Korea

a r t i c l e i n f o

a b s t r a c t

Article history: Received 15 August 2018 Received in revised form 2 December 2018 Accepted 31 December 2018 Available online 4 January 2019

The increasing interest in portable and consumable electronics demands efficient and low-cost energy storage devices with excellent energy storing capacity. From this point, a high mass loading of the active electrode material on three-dimensional current collectors is favorable for obtaining a higher energy storing capacity for the corresponding device. The present study demonstrates an ideal example of highperformance aqueous asymmetric supercapacitors (SCs) using a-MnO2 nanowires as the positive electrode and h-WO3 nanorods as the negative electrode, respectively. Initially, one-dimensional (1D) nanostructures composed of a-MnO2 and h-WO3 are prepared on carbon cloth by a conventional hydrothermal method. The prepared a-MnO2 and h-WO3 with a high mass loading of 4.9 mg/cm2 and 5.8 mg/cm2, respectively, show excellent electrochemical features in aqueous Na2SO4 electrolyte in the positive and negative potential regions. A high-performance asymmetric SC is developed with uniquely engineered electrodes, which exhibits the excellent electrochemical performance in an extended potential window of 1.4 V and with excellent cycling stability of ~135% after 7500 cycles with a volumetric capacitance of 350 mF/cm3 and energy density of 0.095 mWh/cm3. This authentic scheme may offer new opportunities for developing asymmetric arrangements for energy storage devices in various portable electronic systems with a high mass loading. © 2019 Elsevier Ltd. All rights reserved.

Keywords: Asymmetric supercapacitor High mass loading Tungsten oxide

1. Introduction Over the past decades, the increment in environmental problems and forthcoming exhaustion of resources have become major problems. Therefore, the development of clean and renewable energy sources is necessary to meet the energy demands. Fortunately, many governments and research institutes have already predicted an upcoming fuel shortage and been concerned about finding an approach to solve these problems. Through these efforts, many types of energy sources have been investigated in the past few years, such as solar energy, wind power, and geothermal heat [1]. These types of energy sources can reduce the air pollution and solve environmental problems. However, these energy sources are not available at all the times, and hence, it is necessary to develop relevant energy storage devices having high energy storing capacities for storing these energies [2,3]. In this view, two major electrochemical energy storage devices

* Corresponding author. E-mail address: [email protected] (D.-H. Kim). https://doi.org/10.1016/j.electacta.2018.12.187 0013-4686/© 2019 Elsevier Ltd. All rights reserved.

have attracted attention in the past few years, namely, supercapacitors (SCs) and lithium-ion batteries (LIBs), owing to their remarkable features [4e6]. Specifically, SCs are well-known electrochemical energy storage devices because they have a higher energy density than the conventional capacitors and higher power density than LIBs. Moreover, SCs have many advantages including high charge/discharge rate and long cycling stability, which allow their application to other types of SC devices such as flexible electrochromic SC hybrid electrodes based on tungsten oxide films and silver nanowires [7,8]. However, it is crucial to improve the performance of SC devices in terms of the energy density and operating voltage to satisfy the energy demand for the various next-generation electronic devices without sacrificing the features of power density and long-term stability [2,5]. The energy density of SCs is calculated using the equation, E ¼ 12 CV2, where E is the energy density, C is the specific capacitance, and V is the operating voltage. Based on this equation, the energy density can be improved by increasing the capacitance and/or cell voltage [9]. Hence, an effective method is to fabricate asymmetric SCs using different electrode materials with different potential windows.

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Most importantly, when fabricating asymmetric SCs, we should consider the electrochemical properties of the active electrode materials, which implies that both the electrode materials have to work in the same electrolyte and the mass and charge balances should be considered for increasing the operating voltage of the device. Many researchers have used carbon-based materials as negative electrodes owing to their many advantages of low cost, hierarchical porous channels, high electrical conductivity, and better cycling stability [10e12]. However, the lower capacitance of carbon-based materials limits the resultant electrochemical performance of asymmetric SCs in accordance with the overall 2 1 ¼ 1 1 capacitance density equation (Ctotal Cpositive þ Cnegative ; on a cm basis) [13]. Therefore, transition metal oxides (TMOs) having a higher theoretical capacitance compared with carbon-based materials have been studied as the negative electrode for improving the electrochemical performance of asymmetric SC devices [14,15]. Among these TMOs, tungsten oxide (WO3) is a promising negative electrode material for asymmetric SC devices because it has a high theoretical capacitance and can achieve a high operating voltage compared with carbon-based asymmetric SC devices [16e18]. Recently, many researchers have synthesized nanostructured WO3 for application in SCs or LIBs. For example, J. Yang et al. [19] produced hierarchical chrysanthemum-like WO3$0.33H2O by hydrothermal treatment and examined the electrochemical performance of LIBs. H. Peng et al. [20] synthesized WO3 nanorod bundles and showed a capacitance retention of approximately 88% after 1000 cycles in a CNF//WO3 asymmetric SC device. The electrochemical performance of WO3 has enhanced gradually through these efforts. Nevertheless, till now, it is not sufficient to simply meet the requirement of the device performance. To solve this problem, we need to approach in terms of the structures, materials, and mass loading. We are also considering combining these factors appropriately, such as by modification of the structures using additives, different synthesis methods, and the interaction of each material and so on. 1D nanostructure (nanowire, nanorod, and nanotube) arrays built on conducting substrates, such as FTO, ITO, Cu foil, carbon cloth (CC), and graphene, have been studied to optimize electrode structures. These types of structures can provide the transport pathway for ions and electrons because each nanorod/nanowire directly contacts the current collector, which modifies the reduced resistance [21,22]. Additionally, a high mass loading is an important key for improving the areal capacitance and energy density of the device. Clearly, large amounts of active materials on the electrode modify the specific capacitance because the electrode materials in the bulk do not act as active materials during the chargeedischarge process and only the surface materials can participate in it. However, SCs with a low amount of mass loading lead to a poor energy density of the device. From this perspective, it is a challenge to achieve maximum utilization of the active materials on the electrode with a high mass loading [23,24]. In this work, we prepared h-WO3 nanorods with 5.8 mg/cm2 and a-MnO2 nanowires with 4.9 mg/cm2 directly grown on CC for the fabrication of asymmetric SC devices. The maximum areal capacitance of the prepared h-WO3 was 377.14 mF/cm2 at a current density of 4 mA/cm2. The asymmetric SC device was assembled using h-WO3 as the negative electrode and a-MnO2 as the positive electrode in 1 M Na2SO4 electrolyte. This device exhibited a wide applied potential range of up to 1.6 V, and the optimal applied potential was 1.4 V. In this range of applied potential, the asymmetric SC device showed a maximum volumetric capacitance of 350 mF/cm3 at a current density of 1 mA/cm2 and energy density of 0.095 mWh/cm3 at power density of 7 mW/cm3 with a promising electrochemical stability.

2. Experimental 2.1. Materials Sodium tungstate dihydrate (Na2WO4$2H2O), ammonium sulfate ((NH4)2SO4), oxalic acid (C2H2O4), sodium sulfate (Na2SO4), potassium permanganate (KMnO4), and manganese sulfate monohydrate (MnSO4$H2O) were purchased from Sigma-Aldrich. Hydrochloric acid (HCl) was purchased from DAEJUNG. CC (W0S1002, 360 mm) was purchased from FC international. All the chemicals were used without any purification.

2.2. Preparation of h-WO3 on carbon cloth The h-WO3 nanorod structures were synthesized on CC by the hydrothermal method. First, 25 mM sodium tungstate dihydrate (Na2WO4$2H2O) was dissolved in deionized water (80 mL) with stirring for 15 min. Then, 3 M HCl aqueous solution was slowly added to the solution until the pH value of the solution reached 1.15e1.2. After this step, the color of the solution was changed to yellowish. Subsequently, 35 mM oxalic acid (H2C2O4) was added to the prepared solution and diluted to 200 mL under stirring for 20 min, in order to obtain a homogeneous H2WO4 solution. Next, 40 mL of the prepared H2WO4 solution was transferred to a 60 mL Teflon-lined stainless steel autoclave, and 3 g ammonium sulfate was added to it. Prior to the hydrothermal treatment, a piece of CC (2  4 cm2) was cleaned sequentially with acetone, ethanol, and deionized water, and then dried at 60  C for 1 h. Each step was carried out for 15 min. The as well-cleaned CC was immersed in the autoclave and sealed and maintained at 180  C for 12 h. After the hydrothermal process was completed, the Teflon-lined stainless steel autoclave was cooled down to the room temperature. Subsequently, we allowed the reactor to completely be cool, in order to obtain a high-mass-loaded sample. As the temperature outside the reactor is different from that inside it, the inner temperature of the reactor would be lower. The substrate was taken out from the autoclave, carefully rinsed with deionized water several times (2e3 times), and dried at 60  C in the oven under ambient air for 6 h. Last, the as obtained CC was annealed at 400  C for 3 h to obtain h-WO3. The mass of the h-WO3 formed on the CC was measured by a sensitive weigh balance (0.1 mg). To obtain the correct mass loading value of h-WO3, the measurement of the mass was conducted 10 times using a micro-balance, and the average mass loading was 5.8 mg/cm2.

2.3. Preparation of a-MnO2 on the carbon cloth The MnO2 nanowire structures were synthesized on CC by the hydrothermal method. First, 60 mM potassium permanganate (KMnO4) and 21.5 mM manganese sulfate monohydrate (MnSO4$H2O) were dissolved in deionized water (40 mL) with stirring for 15 min. Then, this solution was transferred to a 60 mL Teflon-lined stainless steel autoclave, and the as prepared CC was immersed in the autoclave and sealed and maintained at 140  C for 24 h to obtain the nanowire structure. After this process was completed, the Teflon-lined stainless steel autoclave was cooled to room temperature and left until it was completely cooled to obtain a high-mass-loaded sample. The as obtained CC was removed from the autoclave and carefully rinsed with deionized water several times (2e3 times). Then, it was dried at 60  C for 6 h in air. The mass loading of a-MnO2 on the CC was estimated 10 times using a precise balance, and the average mass loading was 4.9 mg/cm2

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2.4. Fabrication of a-MnO2//h-WO3 asymmetric SC device

3. Results and discussion

For fabricating an asymmetric SC device, it is necessary that two electrodes with different potential windows work in the same electrolyte. h-WO3 and a-MnO2 are well-known negative and positive electrodes, respectively, in Na2SO4 electrolyte. The asymmetric SC device was fabricated by combining h-WO3 (1  1 cm2) as the negative electrode and a-MnO2 (1  1 cm2) as the positive electrode with 1 M Na2SO4 electrolyte. A cellulose paper acting as the separator was immersed in 1 M Na2SO4 electrolyte for 1 h and placed between the two electrodes. Last, the asymmetric SC device was assembled using a Swagelok cell.

3.1. Preparation and characterization of the h-WO3 nanorods on the carbon cloth Tungsten trioxide is usually synthesized in the form of monoclinic (m-WO3) or hexagonal (h-WO3). Both the structures are composed of one tungsten atom and six oxygen atoms, but their arrangement is different from each other. In the h-WO3 case, WO6 are arranged in an octahedral structure, sharing the (001) plane of oxygen atoms at the corner [25]. This crystal lattice structure can provide many ions with more space for penetration during chargeedischarge, which affects the increment of the electrochemical performance compared with m-WO3. The reaction mechanism of forming WO3 is as follows [21]:

2.5. Electrochemical measurements The electrochemical measurement was performed by an automatic battery cycler (WBCS3000) in a typical three-electrode system using 1 M Na2SO4 aqueous solution as the electrolyte. In this system, h-WO3/CC (1  1 cm2) and a-MnO2/CC (1  1 cm2) were used respectively as the working electrode without any current collector. A platinum foil and saturated calomel electrode (SCE) were used as the counter and reference electrodes, respectively. Cyclic voltammetry (CV) and galvanostatic chargeedischarge (GCD) measurements for each sample at different scan rates and current densities. Electrochemical impedance spectroscopy (EIS) was characterized by using an impedance workstation in the range of 10 mHz to 10 kHz. The specific and areal capacitance were calculated using the following equation (Eq. (1)):

Cs ¼

Id  Td

; Ca ¼

DV  m

Id  Td

DV  S

(1)

where Cs is the specific capacitance, Ca is the areal capacitance, Id is the discharging current, Td is the discharging time, DV is the potential window, m is the mass loading of the active materials, and S is the working area of the electrodes. The energy density and power density were calculated using the following equations (Eq. (2) and (3)):

  2 2 1 C  V max  V min E¼  2 3:6 P¼

E  3600 Td

(2)

(3)

where C is the capacitance, Vmax and Vmin are the maximum and minimum potentials during charge and discharge process, respectively, and Td is the discharge time.

2.6. Characterization The h-WO3 and a-MnO2 samples were characterized via fieldemission scanning electron microscopy (FE-SEM; JEOL JSM7500 F) and transmission electron microscopy (TEM, FEI TECNAI G2 F20) at the Korean Basic Science Institute (KBSI, Gwangju Center) to investigate the structures and morphologies of the electrode materials. The crystal structure and oxidation state were investigated via X-ray diffraction (XRD; X'Pert Pro using CuKa) analysis and Xray photoelectron spectroscopy (XPS, VG Multilab 2000), respectively.

Na2 WO4 þ 2HCl þ nH2 O / H2 WO4 $nH2 O þ 2NaCl H2 WO4 $nH2 O

/

Hydrothermal

WO3 þ ðn þ 1ÞH2 O

The structure and morphology of the material depend on different preparative parameters such as the temperature, reaction time, precursors, and complexing or capping agent (e.g., Na2SO4, NaCl, Li2SO4). We have used the ammonium sulfate as an agent to form 1D h-WO3 on CC. Herein, NHþ 4 ions were used as the stabiliþ zation agent and SO2 4 ions were used as the capping agent. As NH4 are cations, they usually have a large radius and mass compared with small cations such as Liþ and Kþ. This property can lead to the easy penetration of the ionic groups, causing SO2 4 to easily act as a capping agent to adsorb onto the surface parallel to the c axis of the h-WO3. After this, the 1D nanostructure (such as nanorod, urchinlike sphere, and others) can be formed [26]. After the hydrothermal process, the sample was annealed at 400  C for 3 h to form the h-WO3 structure. The morphology and crystal structure of the prepared WO3 can be affected by the annealing temperature. To obtain high-purity h-WO3, an appropriate annealing temperature should be considered, which can effectively transform the formation of h-WO3 without the aggregation of the nanorods and removal of the hydrate on the surface of WO3. According to a previous report, the crystal phase becomes converted from an orthorhombic to an anhydrous hexagonal at 400  C, and the annealing temperature further increased over the 500  C, which produced the stable monoclinic WO3 with aggregation. These phase transformations that depend on the annealing temperatures proceed from the break of the mother phase framework, chemical composition change, and formation of new bonds in sequence, which result in a new stable structure [27]. The as obtained sample was characterized via different techniques to identify the structure and morphological features of the material. The XRD pattern of the prepared material is presented in Fig. 1a. We note that each peak corresponds with those of hexagonal WO3 (JCPDS 01-075-2187), and the crystalline nanorod structures were well-formed through the sharp peaks [27]. We observe one additional peak in the XRD pattern at 2q z 25 , which originates from the CC as a substrate (denoted by D). Additionally, some weak peaks exist in the XRD pattern except for h-WO3 and CC peaks, which are related to the transparent tape that was used to fix the sample on the disc for XRD analysis in order to obtain correct information concerning hexagonal WO3 (inset of Fig. 1a). Further to confirm the oxidation state of the elements present in the material, XPS was performed and the corresponding results are presented in Fig. 1b. The broad-scan XPS spectra for h-WO3 nanorod arrays on the CC show the three prominent peaks for W, O, and C with atomic percentages of 20.1%, 64.64%, and 15.26%, respectively. The

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Fig. 1. (a) XRD pattern of h-WO3 on carbon cloth. The transparent tape and carbon cloth (inset of (a)), (b) XPS pattern of h-WO3 nanorod arrays on carbon cloth. The narrow XPS spectra of W 4f (c) and O 1s (d), respectively.

elemental carbon originates from the CC. Fig. 1c shows the narrow XPS spectra of W 4f. The binding energies of the doublet peaks are 38.48 eV and 36.33 eV for 4f5/2 and W 4f7/2, respectively. These doublet energies correspond to W6þ. Additionally, there are two doublet peaks with lower binding energies, which originate from 4f5/2 and W 4f7/2 of W5þ oxidation state. As shown in Fig. 1d, there are two peaks in O 1s narrow XPS spectra. One peak is located at 531.0 eV, which corresponds to the O2 combined with W ions. The additional peak of lower energy is concerned about the hydroxyl groups on the surface of WO3 [26,28,29]. The h-WO3 nanorod arrays grown on the CC were characterized via FE-SEM. According to Fig. 2a, the h-WO3 nanorod arrays are covered on the CC uniformly and formed closely with each other. This structure can cause the fast intercalation of ions and transportation of electrons, which results in a reduction of the surface resistance. Fig. 2b and c represent the h-WO3 nanorods well-

divided on the CC. Well-divided nanorods have a large surface area, which contributes to enhancing the electrochemical performance. TEM was performed to obtain more detailed information about the morphology and structures of the h-WO3 nanorods. In Fig. 2d, the as obtained h-WO3 nanorods have a diameter of ~80 nm and length of ~3.5 mm. This shape corresponds to the SEM images of the h-WO3 nanorods in Fig. 2aec. The energy dispersive spectrometer (EDS) mapping analysis of h-WO3 is shown in Fig. 2e. We confirm that the nanorod of h-WO3 is uniformly composed of W and O on the surface. Fig. 2f displays the TEM of h-WO3 nanorod, and an interplanar distance of 0.38 nm is observed. In addition, the selected area electron diffraction (SAED) pattern (inset of Fig. 2f) shows a single crystal structure of h-WO3. In the inset of Fig. 2f, the lattice distance is [0.38] nm from the TEM image; this result corresponds to the [001] crystal plane of h-WO3 (JCPDS 01-075-2187) [27]. The electrochemical properties of the as obtained h-WO3 were examined by CV and GCD in a three-electrode system in 1 M Na2SO4 aqueous solution. Typically, the h-WO3 based SCs show a good electrochemical performance in the negative potential window with Na2SO4 aqueous solution. Fig. 2g presents the curves of h-WO3 at different scan rates (5e100 mV/s) with a potential window 0.8 to 0 V/SCE. We can see that all the shapes of the CV curves at different scan rates are near-rectangular, which is similar to electric double-layer SCs [30]. As the scan rates increase, the current densities also increase, which indicates that the active materials are utilized effectively throughout the electrochemical reaction and this material is good for SCs. During the electrochemical reactions, the sodium ions (Naþ) are adsorbed by the dangling oxygen atoms to form a cation layer. After forming the layer, this attracts sulfate ions (SO2 4 ) to forms an anion layer. The reaction mechanism of the redox is as follows [26,31]:

WO3 þ xe þ xNaþ 4Nax WO3 Fig. 2h presents the GCD curves of the h-WO3 nanorods at different current densities. The specific capacitance and areal capacitance are calculated using the above equation (Eq. (1)). The areal capacitance (specific capacitance) value of h-WO3 at different current densities is 377.14 (65.02), 271.43 (46.8), 214.29 (36.95), 190 (32.76), and 160 mF/cm2 (27.59 F/g) at 4, 5, 6, 7 and 8 mA/cm2, respectively, in Fig. 2i. The areal capacitance of the prepared h-WO3 is higher than in a previous report for WO3x/MoO3x core/shell nanowires on a carbon fabric (~216 mF/cm2 at 2 mA/cm2) [32], WO3/PANI (200 mF/cm2) [33], and W18O49 (25e17.5 mF/cm2) [34]. The h-WO3 nanorods on the CC show outstanding capacitances because the 1D nanorod structures can provide fast electron transfer pathways and a high surface area for the supercapacitive reaction. Moreover, the CC as a high-conductivity current collector has the positive effect of the enhancement of the electrochemical performance [21]. 3.2. Preparation and characterization of the a-MnO2 nanowires on the carbon cloth

Fig. 2. (aec) FE-SEM images of h-WO3 on carbon cloth at different magnifications. (dee) TEM image of h-WO3 nanorod and its EDS mapping, respectively. (f) HRTEM image of h-WO3 nanorod and SAED pattern in inset of (f). (g) Cyclic voltammetry (CV) of h-WO3/CC at various scan rates from 5 mV/s to 100 mV/s. (h) Galvanostatic chargeedischarge (GCD) of h-WO3/CC at various current densities from 4 mA/cm2 to 8 mA/cm2. (i) Specific capacitance (left) and areal capacitance (right) of h-WO3/CC according to the current density.

To assemble the asymmetric SC device, two different electrodes are required to have different operating potential windows in the same electrolyte. MnO2 is a well-known positive electrode, which works in the Na2SO4 electrolyte. With this motivation, to assemble a high-energy asymmetric SC device, we have prepared a-MnO2 nanowires on CC substrate by the hydrothermal method without using any binder or additive. Fig. 3a presents the a-MnO2 nanowire structure on the CC. In the complete spectrum, the peaks correspond to the index of a-MnO2 (JCPDS 44-0141) [35]. One additional peak in the XRD pattern is owing to the CC peak (denoted by D). The

S.-H. Ji et al. / Electrochimica Acta 299 (2019) 245e252

Fig. 3. (a) XRD pattern of a-MnO2 on carbon cloth. (b) XPS pattern of a-MnO2 nanowire arrays on carbon. The narrow XPS spectrum of Mn 2p (c) and O 1s (d), respectively.

as prepared a-MnO2 was also estimated via XPS analysis, the results of which are displayed in Fig. 3b. Clearly, Mn, O, and C are detected in the XPS, indicating the purity of a-MnO2. Fig. 3c provides the narrow XPS spectrum of Mn 2p. Herein, the binding energies of the doublet peaks are exhibited at 653.9 eV and 642.4 eV for Mn 2p1/2 and Mn 2p3/2 of Mn4þ in MnO2, respectively. The narrow spectrum of O 1s is in Fig. 3d. There are three different peaks at 529.9 eV, 531.3 eV, and 532.3 eV, which correspond to the oxygen bonding to the tetravalent oxide, hydrated trivalent oxide, and residual structure water, respectively [36,37]. For observation of the morphology of the as prepared a-MnO2 on the CC, the sample was investigated using SEM at different magnifications, and the results are given in Fig. 4aec. Fig. 4a and its inset show that the a-MnO2 nanowires are uniformly grown on CC and are well-interconnected. We also confirm a similar morphology at higher magnifications in Fig. 4b and c. As shown in the inset of

249

Fig. 4a, a-MnO2 nanowires are well-grown on CC and wellinterconnected, thereby enhancing the electrochemical performance due to rapid ion and electron transfer through the provided pathways from the interconnected nanowires. To obtain detailed information, the as prepared a-MnO2 was investigated by TEM, with the results in Fig. 4def. Fig. 4d shows that the diameter and length of a-MnO2 are ~100 nm and ~1.2 mm, respectively. Fig. 4e displays the EDS mapping analysis of a-MnO2, and we confirm that Mn and O are distributed uniformly over the a-MnO2 nanowire. Fig. 4f exhibits the TEM of the a-MnO2 nanowires. The SAED pattern (inset of Fig. 4f) shows a single crystal structure of a-MnO2. In Fig. 4f, the lattice distance is [0.23] nm; this result corresponds to the [211] crystal plane and XRD result of a-MnO2 (JCPDS 44-0141) [35]. The electrochemical properties of the as obtained a-MnO2 were examined via CV and GCD in a three-electrode system in 1 M Na2SO4 aqueous solution. a-MnO2 typically operates in the positive window with Na2SO4. Fig. 4g shows the CV curves of the as prepared a-MnO2 at different scan rates. The CV curves are nearrectangular, regardless of the scan rates, indicating excellent supercapacitive behavior [38]. The GCD measurements were performed at different current densities of 1e5 mA/cm2 with potential window of 0e0.8 V/SCE, as shown in Fig. 4h. The specific capacitance and areal capacitance are calculated using the equation (Eq. (1)) stated earlier. The areal capacitance (specific capacitance) values of a-MnO2 are 235 (47.96), 180 (36.73), 142.5 (29.08), 120 (24.49), and 106.25 mF/cm2 (21.68 F/g) from 1 to 5 mA/cm2, respectively, as presented in Fig. 4i. The prepared a-MnO2 displays good areal capacitances and shapes, which are attributed to the nanostructures of a-MnO2. Typically, two mechanisms are proposed for the chargeedischarge reaction. First, many cations (C ¼ Hþ, Liþ, Naþ, and Kþ) of the electrolyte can adsorb on/desorb from the surface of manganese dioxide:

ðMnO2 Þsurface þ C þ þ e 4ðMnOOCÞsurface The second mechanism is the intercalation/deintercalation of the electrolyte cations in the bulk of manganese dioxide.

ðMnO2 Þbulk þ C þ þ e 4ðMnOOCÞbulk Generally, the surface adsorption/desorption reaction occurs well in amorphous MnO2, whereas the intercalation/deintercalation reaction occurs well in crystalline MnO2. In this view, well-formed interconnected nanowire structures of MnO2 would have an effect of enhancing the ion transfer of the electrolyte [35,39].

3.3. Fabrication and performance evaluation of a-MnO2//h-WO3 asymmetric SC device

Fig. 4. (aec, inset of (a)) FE-SEM images of a-MnO2 on carbon cloth at different magnifications. (dee) TEM image of the a-MnO2 nanowire and its EDS mapping. (f) HRTEM image of a-MnO2 nanowire and its SAED pattern in inset of (f). (g) Cyclic voltammetry (CV) of a-MnO2/CC at various scan rates from 5 mV/s to 100 mV/s. (h) Galvanostatic chargeedischarge (GCD) of a-MnO2/CC at various current densities from 1 mA/cm2 to 5 mA/cm2. (i) Specific capacitance (left) and areal capacitance (right) of aMnO2/CC according to the current density.

For examining the feasibility of combining h-WO3 and a-MnO2 in a device, an asymmetric SC device was fabricated using h-WO3 as the negative electrode and a-MnO2 as the positive electrode with 1 M Na2SO4 aqueous electrolyte using a Swagelok cell (Fig. 5). First, to determine the optimized operating potential of the fabricated device, both the electrodes were examined via electrochemical measurements. The as obtained h-WO3 nanorod electrode was measured in the range of potential 0.8 to 0 V/SCE, whereas the aMnO2 nanowire electrode was measured in the positive potential window 0e0.8 V/SCE at a scan rate of 100 mV/s (Fig. 6a). Therefore, the fabricated asymmetric SC device using h-WO3 and a-MnO2 can work in an extended potential range of up to 1.6 V/SCE in 1 M Na2SO4 aqueous electrolyte. Fig. 6b shows the GCD curves of h-WO3 and a-MnO2 at 4 mA/cm2 qþ ¼ q- is the charge balance relation of

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Fig. 5. Schematic of the preparation of the assembled a-MnO2//h-WO3 asymmetric SC device: (I) preparation of h-WO3, (II) preparation of a-MnO2 (III) fabrication of the aMnO2//h-WO3 asymmetric SC device.

Fig. 6. (a) CV curves of both the electrodes with different potential windows at the scan rate of 100 mV/s in 1 M Na2SO4 electrolyte. (b) GCD curves of both the electrodes at the current density of 4 mA/cm2 (c) CV curves of the assembled a-MnO2//h-WO3 asymmetric SC device collected in different applied potential windows at 100 mV/s (d) CV curves of the device at various scan rates and (e) GCD curves at various current densities, and (f) Specific capacitance (left) and volumetric capacitance (right) according to the current density.

the asymmetric SC device. Each electrode of the device can store the charge by the following equation [40]:

q ¼ C  DE  m

scan rates and current densities. The CV curves of the device exhibit near-rectangular shaped, regardless of the scan rates, indicating desirable fast charge/discharge properties of the device and ideal capacitive behavior [42]. The GCD curves display the IR drop in the initial part of the discharge curve, which is related to the internal resistance of the electrode. The IR drop increases with increasing current density, indicating that the electrolyte ions could not completely access the active sites of the electrode during the electrochemical reaction process. Additionally, the asymmetric SC device achieves a coulombic efficiency of 92.01%, suggesting an excellent supercapacitive feature [43]. Fig. 6f displays the specific capacitance and volumetric capacitance of the device with varying current densities. The maximum specific and volumetric capacitance for the a-MnO2//h-WO3 asymmetric SC device were 3.27 F/g and 350 mF/cm3 at a current density of 1 mA/cm2, respectively. The long-term cycling stability is a prime requirement of energy storage devices to control the maintenance cost of the electronic gadgets. Here we have measured the cycling stability of the asymmetric device in terms of the current density to examine the working ability of the asymmetric device at different current densities. Fig. 7a shows the capacitance retention measurement at different current densities from 2 to 6 mA/cm2 at the applied potential of 1.4 V. The first 50 cycles were performed at 2 mA/cm2, following which the measurement was performed at different current densities from 3 to 6 mA/cm2 with an interval of 50 cycles. Last, the capacitance retention was measured at 2 mA/cm2. Fig. 7a exhibits the areal capacitance retention of more than ~99% after 300 cycles, indicating the fabricated device shows excellent rate capability. The continuous cycling stability of the fabricated device was evaluated at the current density of 4 mA/cm2 with an operating window of 1.4 V for 7500 cycles because long-term cycling stability is necessary for application to a commercialized device [44]. The areal capacitance gradually increases until 1200 cycles, after which the capacitance retention is maintained as seen in Fig. 7b. The increment in the capacitance is related to the long activation period in the initial step, which is a result of the high mass loading and rough morphology. During the redox reaction of the device, the active materials in the electrode will be activated by the intercalation/deintercalation of many electrolyte ions. This process provides the opportunity to form more active sites, which in turn

(4)

where C is the specific capacitance, E is the potential range of the charge/discharge, and m is the mass of the electrode. To obtain the information of the mass balance with each electrode, we calculated the value using the following equation [40]:

mþ C  DE ¼ m Cþ  DEþ

(5)

Based on the above values of Cs and potential window in GCD for the h-WO3 and a-MnO2 asymmetric SC device, the optimal mass balance should be m(h-WO3)/m(a-MnO2) ¼ 1.49. To determine the optimum applied potential range of the fabricated device, we investigated the electrochemical performance at different applied potentials. Fig. 6c displays the CV curve of the a-MnO2//h-WO3 asymmetric SC device at various applied potentials from 0.8 to 1.6 V. The optimal applied potentials of the a-MnO2//h-WO3 asymmetric SC device is 1.4 V. We confirm that the CV curves maintain their shape up to 1.4 V and that further increasing the voltage disturbs the CV profile by suddenly increasing the current density. This implies that the fabricated device can operate appropriately up to 1.4 V in terms of the reversible electrochemical reactions [40,41]. Fig. 6d and e shows the CV and GCD at various

Fig. 7. Capacitance retention measurement as a function of various current densities (a), constant current density of 4 mA/cm2 (b) at applied potential of 1.4 V. (c) Nyquist plot of the assembled device. (d) Ragone plots of the a-MnO2//h-WO3 asymmetric SC device.

S.-H. Ji et al. / Electrochimica Acta 299 (2019) 245e252

provides many ions participating in the redox reaction. Additionally, the gradual increment in the capacitance until 1200 cycles is related to the improvement in the surface wettability and dissolution of the active materials on the surface [45]. EIS measurements were performed to study the capacitive properties and ionic resistance of the a-MnO2//h-WO3 asymmetric SC device. Fig. 7c shows the Nyquist plot from 10 mHz to 10 kHz. The Nyquist plot is divided into three parts such as equivalent series resistance, charge-transfer resistance, and Warburg impedance. The equivalent series resistance (ESR, Rs) is exhibited in the highfrequency region on the real axis, which is related to the inherent resistance of the materials, bulk resistance of the electrolyte, and interfacial resistance between the electrode and electrolyte. Second, the semicircle in the medium frequency region indicates the charge-transfer resistance (Rct) that is associated with the charge transfer between the electrode and electrolyte interfaces. Last, the straight line in the low-frequency region represents the Warburg impedance (Zw) relating to the ionic diffusion and charging of the electrode surface [46]. The observed values of Rs and Rct for the asymmetric SC device are 1.61 U/cm2 and 4.92 U/cm2, respectively. The low values of Rs and Rct are indicative of a good electrochemical response between the electrode materials and electrolyte ions [47]. The power density and energy density are important indicators of the device performance. These values were calculated using Eqs. (2) and (3). The a-MnO2//h-WO3 asymmetric SC device achieves the highest energy density of 0.095 mWh/cm3 at power density of 7 mW/cm3. Fig. 7d is the Ragone plot of the a-MnO2//h-WO3 asymmetric SC device. Although the assembled device has an applied potential of only 1.4 V, it has a relatively high energy density compared with previous SC devices, for example, TiN solidstate SC device (0.05 mWh/cm3) [48], TiO2@C coreeshell NWs SC device (0.011 mWh/cm3 at a power density of 0.019 W/cm3) [49], and ZnO@ZnO-doped MnO2 SC device (0.04 mWh/cm3 with a power density of 2.44 mW/cm3) [50]. These results are attributed to many factors, such as the nanostructure which was directly grown on the substrate with high mass loading, and intrinsic properties of both electrode materials. As mentioned earlier, the 1D nanostructure of both electrodes can provide a fast transfer pathway for electron and electrolyte ions with a high surface area for electrochemical reactions, thereby enhancing the performance. Furthermore, both electrode materials that were directly grown on CC as a high-conducting substrate lead to the reduction of resistance, consequently enhancing the electrochemical performance. Simultaneously, the asymmetric SC device consists of a-MnO2 and h-WO3 with a high theoretical capacitance and importantly, h-WO3 as a negative electrode material with high mass loading and high theoretical capacitance compared with carbon-based materials; therefore, it positively influences the device performance. The a-MnO2//h-WO3 asymmetric SCs device displayed excellent electrochemical performance in relation to both the electrode materials (a-MnO2 and h-WO3) and electrolyte. Thus, the excellent electrochemical performance of the asymmetric SC device is because (1) the binder-free synthesis approach leads to reduction in the dead surface and resistance of the binder, (2) 1D nanostructures of both the electrode materials on the conducting substrate directly form fast electron transfer pathways and a high surface area, which contribute to the enhancement of the electrochemical performance, and (3) high mass loading values of both the electrodes have the effect of enhancing the energy density of the device. 4. Conclusion In summary, we prepared h-WO3 nanorod arrays on carbon cloth using a binder-free hydrothermal method. The h-WO3

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nanorods grew well uniformly on the carbon cloth and had a high mass loading of 5.8 mg/cm2. The supercapacitor exhibited a high areal capacitance of 377.14 mF/cm2 at 4 mA/cm2. The asymmetric SC device was fabricated using h-WO3 as the negative electrode and a-MnO2 as the positive electrode in Na2SO4 aqueous electrolyte. Specifically, both the electrodes were prepared directly on carbon cloth with 1D structures such as nanorods and nanowires, which led to the enhancement of the performance through the fast electron transfer pathways, high surface area, and high conductivity of the substrate. The a-MnO2//h-WO3 asymmetric SC device operated at the applied potential of 1.4 V with a volumetric capacitance of 350 mF/cm3 (3.27 F/g) at a current density of 1 mA/cm2. More importantly, the asymmetric SC device displayed an excellent energy density of 0.095 mWh/cm3 at the power density of 7 mW/cm3. Furthermore, the asymmetric SC device exhibited excellent electrochemical stability (135% after 7500 cycles) and rate stability (~99% after 300 cycles), which are suitable for SC devices. These outstanding electrochemical features of the a-MnO2//h-WO3 asymmetric SCs device offer the positive potential for highperformance energy storage application that is small in size and eco-friendly and has a high energy density. Conflicts of interest The authors declare no conflict of interest. Acknowledgments This research was supported by the National Research Foundation of Korea (NRF-2015M3A7B 4050 424). We thank the Korea Basic Science Institute (KBSI) at Gwangju Center for Analysis. References [1] J.R. Miller, P. Simon, Electrochemical capacitors for energy management, Science 321 (2008) 651e652. [2] D.P. Dubal, N.R. Chodankar, D.-H. Kim, P. Gomez-Romero, Towards flexible solid-state supercapacitors for smart and wearable electronics, Chem. Soc. Rev. 47 (2018) 2065e2129. [3] W.-G. Lim, C. Jo, J. Lee, D.S. Hwang, Simple modification with amine- and hydroxyl- group rich biopolymer on ordered mesoporous carbon/sulfur composite for lithium-sulfur batteries, Kor. J. Chem. Eng. 35 (2018) 579e586. [4] W. Wang, W. Liu, Y. Zeng, Y. Han, M. Yu, X. Lu, Y. Tong, A novel exfoliation strategy to significantly boost the energy storage capability of commercial carbon cloth, Adv. Mater. 27 (2015) 3572e3578. [5] C. Ogata, R. Kurogi, K. Hatakeyama, T. Taniguchi, M. Koinuma, Y. Matsumoto, All-graphene oxide device with tunable supercapacitor and battery behaviour by the working voltage, Chem. Commun. 52 (2016) 39191. [6] J. Lee, J.H. Moon, Spherical graphene and Si nanoparticle composite particles for high-performance lithium batteries, Kor. J. Chem. Eng. 34 (2017) 3195e3199. [7] Y. Zhao, J. Liu, Y. Hu, H. Cheng, C. Hu, C. Jiang, L. Jiang, A. Cao, L. Qu, Highly compression-tolerant supercapacitor based on polypyrrole-mediated graphene foam electrodes, Adv. Mater. 25 (2013) 591e595. [8] L. Shen, L. Du, S. Tan, Z. Zang, C. Zhao, W. Mai, Flexible electrochromic supercapacitor hybrid electrodes based on tungsten oxide films and silver nanowires, Chem. Commun. 52 (2016) 6296e6299. [9] K. Xu, W. Li, Q. Liu, B. Li, X. Liu, L. An, Z. Chen, R. Zou, J. Hu, Hierarchical mesoporous NiCo2O4@MnO2 core-shell nanowire arrays on nickel foam for aqueous asymmetric supercapacitors, J. Mater. Chem. 2 (2014) 4795e4802. [10] Z. Fan, J. Yan, T. Wei, L. Zhi, G. Ning, T. Li, F. Wei, Asymmetric supercapacitors based on graphene/MnO2 and activated carbon nanofiber electrodes with high power and energy density, Adv. Funct. Mater. 21 (2011) 2366e2375. [11] Z. Lin, X. Yan, J. Lang, R. Wang, L.-B. Kong, Adjusting electrode initial potential to obtain high-performance asymmetric supercapacitor based on porous vanadium pentoxide nanotubes and activated carbon nanorods, J. Power Sources 279 (2015) 358e364. [12] J. Huang, P. Xu, D. Cao, X. Zhou, S. Yang, Y. Li, G. Wang, Asymmetric supercapacitors based on b-Ni(OH)2 nanosheets and activated carbon with high energy density, J. Power Sources 246 (2014) 371e376. [13] W.G. Pell, B.E. Conway, Peculiarities and requirements of asymmetric capacitor devices based on combination of capacitor and battery-type electrodes, J. Power Sources 136 (2004) 334e345. [14] C. Guan, J. Liu, Y. Wang, L. Mao, Z. Fan, Z. Shen, H. Zhang, J. Wang, Iron oxide-

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