Fabrication of high performance flexible all-solid-state asymmetric supercapacitors with a three dimensional disc-like WO₃/stainless steel electrode

Abstract

Presently, significant attention has been paid towards the rational synthesis of nanostructured anode and cathode electrode materials for assembling high-performance supercapacitors. Despite significant progress being achieved in designing cathode electrode materials, anode electrode materials with high capacitance are hardly investigated. In the present article, a tungsten oxide (WO₃) thin film is prepared on a flexible stainless steel substrate by a wet chemical method and used as an anode electrode to fabricate a flexible asymmetric supercapacitor (ASC). An electrochemical investigation of the WO₃ thin film shows a maximum specific capacitance of 530 F g⁻¹ at 1 mA cm⁻² in a potential window of 0 to −0.8 V in 1 M Na₂SO₄ electrolyte. In addition, a highly energetic, flexible ASC device is assembled using a WO₃ thin film as an anode, a MnO₂ thin film as a cathode and polymer gel as an electrolyte. The as-assembled MnO₂//WO₃ ASC device exhibited a stable electrochemical potential window of 1.8 V and better cycling stability. What's more, the flexible MnO₂//WO₃ ASC device achieves a high specific capacitance of 115 F g⁻¹ with an acceptable specific energy of 52 W h kg⁻¹ at a current density of 3 mA. Hence, the proposed flexible MnO₂//WO₃ ASC device creates one more option for anode materials to develop flexible energy storage devices.

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1 Introduction

The energy storage device is the key component of advanced electronic gadgets (like mobile phones, epapers, laptops, hybrid electrical vehicles, memory backup system, etc.) and defines the dimensions, safety and functioning time of the electronic gadgets. Therefore, researchers and engineers are trying to develop energy storage devices with high energy and power capability with a longer working life. They are also worried about the cost and dimensions of the energy storage devices. Presently, supercapacitors (SCs) have attracted significant attention as promising and complementary energy storage devices owing to their capabilities like excellent power densities, rapid charge–discharge ability and long life times as compared to battery technology.^1–4 To commercialize SCs for practical application, it is crucial to increase the specific energy and the potential limit of SCs without surrendering other features.

The specific energy (SE) of SCs is related to specific capacitance (C_s) of the electrode material and operating potential window (V) of the SC device according to the equation SE = 0.5C_sV². Therefore, the SE of the SCs can be enhanced by increasing either capacitance or operating potential window of the device. The increasing operating potential window is the best way for enhancing the SE of SCs. Basically, two effective ways to enlarge the operating potential window are (i) the use of non-aqueous electrolyte and (ii) assembling asymmetric supercapacitor (ASCs). The non-aqueous electrolyte improves the operating potential window of the SCs but its higher cost, lower ionic conductivity, high ionic radii, toxic nature and inflammability kept a limit on its use for safe SCs. The advance approach to enhance SE is to fabricate the ASCs. The ASCs consists two different electrodes having different operating potential window in an identical electrolyte, which helps to enhance SE of the SCs device. For the last few years, wide varieties of materials have been investigated as electrode material in fabricating ASCs, including carbonaceous materials, polymers and metal oxides.^5,6 Particularly, the metal oxides are more favorable for ASCs due to their higher energy storing capacity, easy preparation, controlled nanostructured morphology and moderate electrochemical stability.⁷

To achieve sufficient electrochemical performance for ASCs, the selection of particular cathode, anode materials and an electrolyte is critical one. Basically, the specific capacitance of ASCs is calculated using the following equation; 1/C_s = 1/C_c + 1/C_a, where, C_s, C_c and C_a are the specific capacitances of SCs device, cathode and anode electrodes respectively. The metal oxides (particularly MnO₂) are well known cathode materials for assembling the ASCs device.⁸ In past, the carbon based materials such as activated carbon (AC),⁹ graphene,¹⁰ carbon nanotubes¹¹ etc. are explored as the negative electrode materials to fabricate the ASCs device, however, their lower capacitance limit the resultant C_s and SE of ASCs device. In order to enhance the SE and C_s of ASCs devices, an emergent need is to find the new alternative anode electrode materials which combines properties of high capacitance and electrical conductivity. The WO₃ holds great promise as a negative electrode to fabricate the ASCs device due to its higher C_s and operating potential window as compared to carbon based materials. In addition, the low cost, large abundance and non toxic nature of WO₃ make it feasible to develop the highly energetic ASCs devices.

In present investigation, WO₃ thin film is used as an anode electrode to fabricate flexible ASCs device. The flexible ASCs device is assembled using the WO₃ thin film as an anode, MnO₂ thin film as a cathode and polymer gel (PVA–LiClO₄) as an electrolyte as well as separator. The nanostructured WO₃ and MnO₂ thin films are grown on flexible stainless steel substrates and to estimate stable potential window of both electrodes, the electrochemical measurements were performed in 1 M Na₂SO₄ aqueous electrolyte before assembling ASCs device.

2 Experimental

2.1 Synthesis of WO₃ thin film

All chemicals used in the present study were AR grade and used without further purification. For the synthesis of WO₃ thin film, 3.67 g tungsten metal powder was dissolved in 40 ml of H₂O₂. The reaction being exothermic, therefore it was conducted in an ice bath. Minimum period of 2 h is required for the complete dissolution of tungsten metal powder in H₂O₂. The solution was further diluted to 280 ml by addition of double distilled water (DDW). After stirring for 30 min, 50 ml of above solution was transferred to a beaker. Well cleaned flexible stainless steel substrate (5 × 5 cm²) was immersed in above solution and bath temperature is maintained at 353 K for 12 h. Greenish-yellow colored film was deposited on substrate after washed several times with DDW. Finally, the film was annealed in air at 673 K for 4 h to improve crystalline structure.

2.2 Synthesis of MnO₂ thin film

MnO₂ film was grown on flexible stainless steel substrate through a chemical bath deposition (CBD) method. In a typical synthesis of MnO₂ thin film, 0.04 g of KMnO₄ was dissolved in 50 ml of DDW. The solution was stirred for 15 min at room temperature, subsequently 0.5 M HCl added to it which forms bath solution. The well cleaned flexible stainless steel substrate (5 × 5 cm²) was immersed in bath solution and maintained at 338 K for 12 h. The black precipitate was formed in the bath and MnO₂ film deposited on the substrate. The MnO₂ coated substrate was washed several times in DDW and dried at room temperature.

2.3 Preparation of PVA–LiClO₄ gel electrolyte

The Na₂SO₄ polymer gel electrolyte was prepared for MnO₂//WO₃ ASCs device to correlate with electrochemical performance of WO₃ and MnO₂ electrodes. Due to the formation of Na₂SO₄ crystallites in PVA, it is hard to prepare PVA–Na₂SO₄ gel electrolyte. Therefore, PVA–LiClO₄ gel is prepared which have advantages like high ionic conductivity, good flexibility and good compact with the electrodes.¹² The PVA–LiClO₄ gel was prepared in the following manner: 6 g of PVA and 1 M LiClO₄ were added in 60 ml of DDW. The solution was stirred at 343 K to obtain a transparent solution. Later, the solution was constantly stirred at room temperature to form clear viscous appearance. This transparent viscous solution was used as a gel electrolyte and a separator in SC device fabrication.

2.4 Fabrication of flexible ASCs device

The WO₃ and MnO₂ thin films (5 × 5 cm²) were used as an anode and cathode electrode, respectively for assembling the ASCs device. In the first step, both electrodes were painted with PVA–LiClO₄ gel electrolyte and kept in an oven at temperature of 60 °C for 12 h to remove the water contents from the gel electrolyte. Further, the anode and cathode electrodes were placed facing each other and pressed under hydraulic pressure of 1 ton to form the MnO₂//WO₃ ASCs device. The schematic of actual fabricated MnO₂//WO₃ ASCs device is shown in Fig. 1.

Fig. 1 The schematic of steps involved in fabrication of flexible MnO₂//WO₃ ASCs device.

2.5 Material characterisations

The crystal structure of prepared films was examined by X-ray diffraction (XRD; Bruker AXS D8) with Cu Kα radiation (λ = 1.5418 A). The X-ray photoelectron spectroscopy (XPS) data was analyzed using ESCALAB 250Xi X-ray photoelectron spectrometer microprobe for identifying oxidation states. The surface morphology was analyzed by using field emission scanning electron microscope (FE-SEM; S-4700, Hitachi) with an attached energy dispersive X-ray spectroscope (Varian, CARY, 300 Conc.). The high-resolution transmission electron microscopy image was obtained using a high resolution JEOL-3010 microscope. The supercapacitive properties of WO₃ and MnO₂ were tested using automatic battery cycler (WBCS3000) in a conventional three electrode system. Electrochemical impedance spectroscopy (EIS) measurement was performed using electrochemical workstation (ZIVE SP 5) to study the interface between electrode and electrolyte.

3 Results and discussion

3.1 Structural, morphological and electrochemical properties of WO₃

The phase structure of WO₃ thin film is identified from XRD pattern shown in Fig. 2(a). All diffraction peaks are well assigned to monoclinic phase of WO₃ (JCPDS card no. 00-005-0363). The strong and intense peaks indicate well crystalline nature of WO₃. The crystalline nature of WO₃ thin film is more favorable for SCs application, as the ordered nature of crystallite provides the easiest path for the electrolyte ions to make the intercalation and deintercalation. More importantly, the crystalline WO₃ is more stable than that of amorphous WO₃.¹³ Furthermore, to calculate the oxidation state of tungsten in WO₃ thin film, XPS measurement was performed. Fig. 2(b) shows the XPS spectrum for W 4f core level. The binding energies of 37.89 and 35.76 eV are attributed to the W 4f_5/2 and W 4f_7/2, respectively, with a spin–orbit separation of 2.13 eV. The observed energy position of the doublet is in good agreement with the literature for W⁶⁺ oxidation state.¹⁴ The results obtained from the XPS studies are in well analogues with XRD study, confirming the formation of WO₃.

Fig. 2 (a) The XRD pattern of WO₃ thin film, (b) XPS spectrum of W 4f core level, (c and d) FESEM images of WO₃ thin film at two different magnifications (2k× and 10k×) and (e) EDAX spectrum of WO₃ thin film with atomic concentration of W and O.

Fig. 2(c and d) shows FESEM images of WO₃ thin film on flexible stainless steel substrate at two different magnifications (2k× and 10k×). It shows the distribution of randomly oriented three dimensional disc-like structures on all over the substrate. At higher magnification (10k×), the three dimensional disc-like structure is decorated with fine WO₃ nanoparticles. These nanoparticles effectively enhance the electroactive surface area for electrochemical reactions. Additionally, the interface of the disc-like structure creates a large number of voids useful for the intercalation and deintercalation process. Fig. 2(e) depicts the EDAX spectrum of WO₃ thin film. The EDAX spectrum reveals representative peaks of tungsten and oxygen elements with atomic percentage of 23.68% for tungten and 76.32% for oxygen, maintaining good stoichiometry. Further to get more detailed information about the microstructural features, TEM and high-resolution TEM (HRTEM) studies are carried out. Fig. 3(a) displays the TEM image of individual WO₃ three dimensional disc-like structure, which supports the corresponding FESEM results. The HRTEM image in Fig. 3(b) shows interplanar spacing of 0.362 nm, corresponds to the characteristic lattice spacing of (200) plane. This confirms that, three dimensional disc-like structures have crystal structure of monoclinic WO₃.²⁹

Fig. 3 (a) TEM and (b) HRTEM (inset shows magnified fringe pattern) images of WO₃ thin film.

The electrochemical features of WO₃ thin film are evaluated from cyclic voltammetry (CV) and galvanostatic charge–discharge measurements in 1 M Na₂SO₄ aqueous electrolyte using standard three electrode setup. The CV curves of WO₃ electrode at various scan rates (5–100 mV s⁻¹) demonstrated in Fig. 4(a) exhibit quasi-rectangular shape, indicating pseudocapacitive behavior. The increased area under the CV curves with scan rate indicates that WO₃ electrode has superior capacitive behavior and fast diffusion of electrolyte ions into it.

Fig. 4 (a) Cyclic voltammetry (CV) curves, (b) variation of C_s as a function of potential scan rate, (c) GCD curves at different current densities, (d) variation of C_s as a function of current densities, (e) Ragone plot and (f) plot of capacity retention to the number of CV cycles (inset shows 1^st to 2000^th CV cycles) for WO₃ electrode.

The C_s is calculated by using following formula:

where, C_s is the specific capacitance, m is the mass of deposited material (1.0 mg cm⁻²), V_max − V_min is the operational potential window, I is the average current for unit area dipped in the electrolyte. The obtained value of C_s 677 F g⁻¹ (see Fig. 4(b)) for the WO₃ electrode in Na₂SO₄ electrolyte is larger as compared to previous report.²¹ The GCD measurement is very significant method for calculating the C_s of electrode at different current densities. Fig. 4(c) illustrates the GCD curves for WO₃ electrode at different current densities ranging from the 1 to 5 mA cm⁻². The discharge curve is characterized by three distinct regions – at the beginning of discharge, the initial drop in voltage is related to internal resistance of electrode material, the linear variation of potential with time corresponds to double layer capacitance due to charge separation at electrode–electrolyte interface and last region gives subsequent potential decay related to the capacitive behaviour of WO₃ electrode.⁵

The C_s from GCD curves is calculated using following formula:

The SE and SP are calculated using the following formulae:

andwhere, C_s is specific capacitance, I_d is discharge current, T_d is the discharge time, ΔV is potential window and V_max and V_min are the maximum and minimum potential. The C_s values calculated from discharge curves as a function of discharge current densities and plotted in Fig. 4(d). The maximum C_s is found to be 530 F g⁻¹ at 1 mA cm⁻² and remains 318 F g⁻¹ even at higher current density of 5 mA cm⁻². The obtained electrochemical performances of WO₃ electrode is more due to its three dimensional disc-like surface morphology, which provides large electroactive surface area, facilitating easy path for electrochemical reactions. The SE and SP are calculated from discharge curves and fitted in Ragone plot shown in Fig. 4(e). The maximum SE of 39.6 W h kg⁻¹ is achieved at SP of 400 W kg⁻¹ at 1 mA cm⁻². The long term cycling performance is the most important requirement for SCs application. The stability test is performed at constant scan rate of 100 mV s⁻¹ for 2000 CV cycles. The 84% capacity retention observed after 2000 CV cycles is shown in Fig. 4(f). Additionally, to get more information about the electroactivity of WO₃ thin film, electrochemical impedance spectroscopy (EIS) analysis is carried out. The Nyquist plot for WO₃ electrode is shown in Fig. 5 which consists of small semicircle arc in the high frequency region and straight line in the low frequency region. The calculated equivalent series resistance (R_s) and charge transfer resistance (R_ct) 2.1 and 2.0 Ω cm⁻², respectively are estimated from Nyquist plot analysis. The lower values of the R_s and R_ct for WO₃ thin film suggest the easier electrochemical reactions at interface of active electrode and electrolyte. Additionally, the straight line in the lower frequency region is related to capacitive behavior of WO₃ electrode.

Fig. 5 Nyquist plot for WO₃ electrode in 1 M Na₂SO₄ electrolyte.

3.2 Structural, morphological and electrochemical properties of MnO₂

The XRD pattern of MnO₂ thin film is shown in Fig. 6(a). The peaks at the angles 2θ of 37.5, 49.8 and 65.1° correspond to the planes (211), (411) and (002), respectively for the tetragonal phase of MnO₂ (JCPDS card no. 44-0141). The peaks marked with ‘Δ’ are attributed to stainless steel substrate. The XPS spectrum for Mn 2p core level is demonstrated in Fig. 6(b). The spin–orbit separation of Mn 2p_3/2 and Mn 2p_1/2 at binding energies of 642.60 and 653.90 eV, respectively is 11.3 eV, which is in consistent with the previous report.²² The Mn exists in the form of Mn⁴⁺ oxidation state. The FESEM images of MnO₂ thin film at two different magnifications (25k× and 50k×) are shown in Fig. 6(c and d). The MnO₂ thin film shows the formation of highly porous nanoflakes morphology, providing the large cavity for electrochemical reaction that enhances the interface of an active electrode material to the electrolyte ions. This significantly improves the rate of intercalation/deintercalation of electrolyte ions in the active electrode material.

Fig. 6 (a) The XRD pattern of MnO₂ thin film, (b) XPS spectrum of Mn 2p core level and (c and d) FESEM images of MnO₂ thin film at two different magnifications (25k× and 50k×).

The CV measurements of MnO₂ electrode are carried out in potential window of 0 to +1 eV at various scan rates (5–100 mV s⁻¹) in 1 M Na₂SO₄ aqueous electrolyte demonstrated in Fig. 7(a). The mass loading for MnO₂ electrode is 0.35 mg cm⁻². The MnO₂ electrode exhibits nearly rectangular CV curves, indicating ideal capacitive behavior. Fig. 7(b) shows the variation of C_s with scan rate. The maximum C_s of 410 F g⁻¹ is achieved at a lower scan rate of 5 mV s⁻¹ and remains 231 F g⁻¹ even at high scan rate of 100 mV s⁻¹. The GCD curves of MnO₂ electrode at different current densities are illustrated in Fig. 7(c). The non-linear behavior of charge–discharge curves suggest the excellent pseudocapacitive feature of MnO₂ electrode. The C_s from discharge curve is calculated to be 343 F g⁻¹ at 1 mA cm⁻² and remains 272 F g⁻¹ at 5 mA cm⁻². The C_s decreases with increasing current density as shown in Fig. 7(d). The EIS analysis is performed to investigate ion transfer feature of MnO₂ electrode. Fig. 7(e) specifies the Nyquist plot which consists of low frequency semicircle arc and high frequency straight line. The observed values of R_s and R_ct from Nyquist plot as 1.74 and 1.25 Ω cm⁻², respectively imply the high ionic conductivity of MnO₂ electrode. Further, to study practical applicability of MnO₂ electrode for SC device fabrication, stability measurement is carried out for 2000 CV cycles. The excellent stability of the electrode observed with 91% capacity retention after 2000 CV cycles is shown in Fig. 7(f) and inset figure shows 1^st to 2000^th CV cycles. The improved capacity retention of positive electrode is observed as compared to negative electrode due to its highly porous and adhesive nanostructured morphology and less dissolution of MnO₂ during cycling.

Fig. 7 (a) CV curves, (b) variation of C_s as a function of potential scan rate, (c) GCD curves at different current densities, (d) variation of C_s as a function of current densities, (e) Nyquist plot and (f) plot of capacity retention to the number of CV cycles (inset shows 1^st to 2000^th CV cycles) for MnO₂ electrode.

3.3 Electrochemical measurements of MnO₂//WO₃ ASCs device

For developing the ASCs device, the basic requirement is that the electrode material should have different operating potential windows. In present case, MnO₂ works within positive operating potential (0 to +1 V per SCE), while WO₃ shows electrochemical performance in negative operating potential window (0 to −0.8 V per SCE) in identical 1 M Na₂SO₄ electrolyte. The different operating potential windows of MnO₂ and WO₃ thin films are suitable for fabricating the ASCs device. Therefore, if these two electrodes are assembled into asymmetric design, it is expected that operating cell potential should be extended up to 1.8 V. Hence, the flexible MnO₂//WO₃ ASC device was constructed using MnO₂ as a positive electrode and WO₃ as a negative electrode with PVA–LiClO₄ gel as separator and electrolyte. The objective behind the smearing polymer gel electrolyte for ASCs device is due to its lot of advantages over the liquid electrolyte like it removes the electrolyte leakage, electrode corrosion and the electrolyte evaporation problem. More importantly, the polymer gel electrolyte based SCs devices are flexible, light weight and easier for packing which make them suitable for advanced electronics applications.

The first important task before assembling the ASCs device is the balancing the charges of both electrodes (q₊ = q₋) for getting the highest electrochemical performance. The stored charges are related to the C_s, operating potential window (ΔE) and mass of the electroactive material by the equation¹⁵

q = C_s × ΔE × m (5)

From the above equation mass balancing will follow the equation

The C_s and operating potential window for both the electrodes are different; hence charges can be balanced by adjusting the mass loading of the electrode material. The optimal mass ratio between positive and negative electrode was found to be 1.35 : 1.02. Further, to establish appropriate operational potential window for MnO₂//WO₃ ASC device, a series of CVs were measured in different potential window from 1.0 to 1.8 V at a fixed scan rate of 100 mV s⁻¹ as shown in Fig. 8(a). The as-assembled MnO₂//WO₃ ASC device exhibits ideal capacitive performance with quasi-rectangular CV curves at the potential window up to 1.8 V. Fig. 8(b) shows the C_s as a function of potential window for MnO₂//WO₃ ASC device. The C_s increases significantly from 13.4 to 34.0 F g⁻¹ with the operational potential window from 1.0 to 1.8 V. Therefore, in order to investigate the electrochemical performance of MnO₂//WO₃ ASC device, potential window of 0.0 to 1.8 V is chosen. Fig. 8(c) gives the CV curves for MnO₂//WO₃ ASC device at different scan rates (5–100 mV s⁻¹). It is observed that, with increasing scan rate, area of CV curve increases. Fig. 8(d) shows C_s and areal capacitance of WO₃ electrode as a function of scan rate. It is seen that capacitance decreases as scan rate increases which may be a consequence of fast diffusion of electrolyte ions at higher scan rate. The maximum C_s and areal capacitance achieved by MnO₂//WO₃ ASC device are 113 F g⁻¹ and 105 mF cm⁻² at 5 mV s⁻¹. The GCD curves at different currents for MnO₂//WO₃ ASC device are illustrated in Fig. 8(e). The GCD curves represent a good electrochemical capacitive behavior and superior reversible redox reactions. Fig. 8(f) illustrates the C_s of ASC device as a function of current. The C_s is decreased from 115 to 99.9 F g⁻¹ at current for 3 to 10 mA, maintaining high rate capability. The decrease in C_s for higher current and scan rate is observed, due to the lower diffusion of charged ions at high current and scan rate. The SE and SP are derived from the discharge curve at different currents. Fig. 8(g) shows the Ragone plot of MnO₂//WO₃ ASC device. The SE of MnO₂//WO₃ ASC device is 52 W h kg⁻¹ at SP of 130 W kg⁻¹, which is higher than the ASCs reported in the literature. For example, CNF//WO₃ (35.3 W h kg⁻¹ at 314 W kg⁻¹),¹⁶ SiC–N–MnO₂//AC6 (30.6 W h kg⁻¹ at 113.92 W kg⁻¹),¹⁷ MnFe₂O₄/graphene//MnO₂/CNT (25.9 W h kg⁻¹ at 225 W kg⁻¹),¹⁸ PANI//WO₃ (41.9 W h kg⁻¹ at 261 W kg⁻¹)¹⁹ and MnO₂//graphene (25.2 W h kg⁻¹ at 100 W kg⁻¹).²⁰ The increased SE of MnO₂//WO₃ ASC device is mainly attributed to synergistic effects between positive and negative electrodes which increases C_s and operational potential window. The comparison of ASC device performance with MnO₂//WO₃ ASC device is summarized in Table 1.

Fig. 8 (a) CV curves at different potential windows (b) variation of C_s with potential windows, (c) CV curves at different scan rates, (d) plot of C_s and areal capacitance versus potential scan rate, (e) GCD curves at different currents, (f) variation of C_s with current, (g) the Ragone plot; the values reported for other ASC device are added for comparison, (h) Nyquist plot and (i) plot of capacity retention versus number of CV cycles (inset shows 1^st to 2000^th CV cycles) for as-assembled flexible solid state MnO₂//WO₃ ASC device.

Table 1 Comparison of asymmetric supercapacitor device performance

Sr. No.	Type of device	Electrolyte	Potential window	Specific capacitance	Energy density	Power density	References
1	Mn3O4/graphene paper//CNT/graphene paper	KCl/PAAK	1.8 V	72.6 F g^−1 at a current density of 0.5 A g^−1	32.7 W h kg^−1	9.0 kW kg^−1	23
2	MnO2//CoSe2	LiCl/PVA	1.6 V	1.77 F cm^−3 at a current density of 1 mA cm^−2	0.588 mW h cm^−3	0.282 W cm^−3	24
3	Carbon Fiber (CF)–Ni(OH)2//CF–CNT	KOH–PVA	1.3 V	—	41.1 W h kg^−1	3.5 kW kg^−1	25
4	TiN@GNSs//Fe2N@GNSs	LiCl/PVA	1.6 V	60 F g^−1 at scan rate of 50 mV s^−1	15.4 W h kg^−1	6.4 kW kg^−1	26
5	MnO2/CNT//CNT	Na2SO4/silica/PVA	1.8 V	56.3 F g^−1 at a current density of 1 A g^−1	5.97 W h kg^−1	0.87 kW kg^−1	27
6	γ-MnS//eggplant derived activated carbon (EDAC)	—	1.6 V	110.4 Fg^−1 at a current density of 1.0 mA	37.6 W h kg^−1	181.2 W kg^−1	28
7	MnO2//WO3	LiClO4/PVA	1.8 V	115 F g^−1 at a current density of 3.0 mA	52 W h kg^−1	130 W kg^−1	Present work

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The EIS study is employed to illustrate the ion transport features of MnO₂//WO₃ ASC device in the frequency range of 100 kHz to 10 mHz by applying a.c. Amplitude of 10 mV. A typical Nyquist plot for MnO₂//WO₃ ASC device is shown in Fig. 8(h). The distinct region provided by Nyquist plot explains the charge transfer process at the electrode–electrolyte interface represented by a high frequency semicircle arc and low frequency straight line. The intercept of semicircle to real axis gives R_s which is the combination of intrinsic resistance of electrode material, ionic resistance of electrolyte and interfacial resistance. The R_ct is associated with the diameter of semicircle. In the lower frequency region a straight line with inclination ∼45° to the real axis is observed which specifies the diffusion of electrolyte ions from surface of electrode to the electrolyte. The R_s and R_ct values from Nyquist plot are 1.02 and 0.22 Ω cm⁻², respectively. The lower R_ct gives indication of larger electroactive surface area provided by both the positive and negative electrodes. This enhances the capacitive performance of device because of very quick redox reactions that allows facile ion and charge transfer in the electrode material. The long-term cyclic stability of ASCs device is also a prime requirement for practical applications. The stability test is performed at fixed scan rate of 100 mV s⁻¹ for 2000 CV cycles. Fig. 8(i) shows capacity retention as a function of cycle number and the CV curves of 1^st to 2000^th cycles are shown inset. The MnO₂//WO₃ ASC device exhibits superior electrochemical stability with 85% capacitive retention after 2000 cycles.

The mechanical flexibility of ASC device is important for its practical application. To test mechanical flexibility of MnO₂//WO₃ ASC device, CV measurements are performed at different bending angles (0° to 180°). Fig. 9(a) shows CV curves of MnO₂//WO₃ ASC device at scan rate of 100 mV s⁻¹ for different bending angles. The shape of CV curve is almost same upto bending angle of 180°, demonstrates that the bending of the device has no significant effect on the electrochemical performance. The capacity retention for different bending angles is shown in Fig. 9(b). The MnO₂//WO₃ ASC device retained 95% of its initial capacitance at bending angle of 180°. The as fabricated ASC device demonstrates remarkable flexibility and good adhesion of electrode material with current collector. More strikingly, the single MnO₂//WO₃ ASC device charged for 20 s by 2.5 V, can light up single red light emitting diode (LED) for 60 s (see Fig. 10), suggesting the better electrochemical features for MnO₂//WO₃ ASC device.

Fig. 9 (a) CV curves of MnO₂//WO₃ ASC device for different bending angles and (b) plot of capacity retention versus bending angles.

Fig. 10 (a, b) digital photographs showing the actual demonstration of single MnO₂//WO₃ ASCs device by glowing the red LED for 60 s.

4 Conclusions

In summary, the pseudocapacitive WO₃ and MnO₂ thin films are prepared on low cost, flexible stainless steel substrate by wet chemical method and used as active electrodes to fabricate the flexible ASCs. The WO₃ electrode exhibits a high specific capacitance of 530 F g⁻¹ at 1 mA cm⁻² in a wide potential window of 0 to −0.8 V. The excellent capacitive behavior and complementary potential window of WO₃ makes it a perfect anode to match with the MnO₂ cathode in ASC device. The MnO₂//WO₃ ASC device exhibits stable electrochemical potential window of 1.8 V, achieving a high specific capacitance of 115 F g⁻¹ and specific energy of 52 W h kg⁻¹ with better cycling stability. The proposed theme of flexible MnO₂//WO₃ ASC device holds great promise for the applications in flexible energy storage device.

Acknowledgements

This work was supported by the Human Resources Development program (No. 20124010203180) of the Korea Institute of Energy Technology Evaluation and Planning (KETEP) Grant funded by the Korea government Ministry of Trade, Industry and Energy and supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT and Future Planning (NRF-2015R1A2A2A01006856).

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