Flexible Asymmetric Solid-State Supercapacitors by Highly Efficient

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Functional Inorganic Materials and Devices

Flexible Asymmetric Solid State Supercapacitor by Highly Efficient 3D Nanostructured #-MnO2 and h-CuS Electrodes Amar Patil, Abhishek C. Lokhande, Pragati A Shinde, and Chandrakant D. Lokhande ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b03690 • Publication Date (Web): 24 Apr 2018 Downloaded from http://pubs.acs.org on April 24, 2018

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ACS Applied Materials & Interfaces

Flexible Asymmetric Solid State Supercapacitor by Highly Efficient 3D Nanostructured α-MnO2 and hCuS Electrodes Amar M. Patila, Abhishek C. Lokhandeb, Pragati A. Shindea, Chandrakant D. Lokhandea* a

Centre of Interdisciplinary Research, D. Y. Patil University, Kolhapur, 416006 Maharashtra, India

b

Department of Materials Science and Engineering, Chonnam National University, Gwangju, 500-757, South Korea.

*Prof. Chandrakant D. Lokhande Phone: + 91-231-2601212 E-mail:- [email protected]

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ABSTRACT The simplistic and economical chemical way has been used to prepare highly efficient nanostructured, manganese oxide (α-MnO2) and hexagonal copper sulfide (h-CuS) electrodes directly on cheap and flexible stainless steel sheets. Flexible solid-state α-MnO2/FSS/PVALiClO4/h-CuS/FSS asymmetric supercapacitor device (ASCs) have been fabricated using PVALiClO4 gel electrolyte. High active surface areas of α-MnO2 (75 m2 g-1) and h-CuS (83 m2 g-1) electrodes contribute to more electrochemical reactions at electrode and electrolyte interface. The ASCs device has a prolonged working potential of + 1.8 V and accomplishes a capacitance of 109.12 F g-1 at 5 mV s-1, energy density of 18.9 Wh kg-1 and long-term electrochemical cycling with capacity retention of 93.3 % after 5000 cycles. Additionally, ASCs device demonstrated by glowing seven white light emitting diodes (LEDs) for more than seven min after 30 s of charging. Outstandingly, real practical demonstration suggests “ready to sell” product for industries. KEYWORDS Asymmetric supercapacitor, Electrochemical cycling stability, Electrodes, Nanospheres, h-CuS, α-MnO2 INTRODUCTION The portable and small supercapacitors (SCs) are essential in assembly of recent energy storage devices

1,2

, as these show mechanical flexibility, light weight, low-cost, and

environment-friendly energy storage, which give applications in portable, wearable and commercialized pocket electronic devices.3 There is a need to upsurge the energy (ED) as well as 2 ACS Paragon Plus Environment

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power (PD) of energy storage devices using different electrode materials and electrolytes, which can run at wide potential range to tune the working potential of energy storage device. The asymmetric design of supercapacitor is an actual way to augment the potential of device. As an impact of potential window of asymmetric supercapacitors (ASCs), it shows higher ED, PD and specific capacitance (Cs) as compared to the corresponding symmetric supercapacitors.4-6 The various combinations of anode and cathode are reported in literature as, MnO2//Fe3O47, CoMoO4/MnO28, FeWO4/MnO29, NiO//α-Fe2O310, NiO//NiCo2O411 etc. The solid electrolyte based SCs holds many advantages such as small size, lightweight, exceptional reliability and a higher operating temperature. The polymer based gel electrolytes offer polymeric chain to carry on redox reaction between the electrolyte-electrode interfaces. For fabrication of ASCs device, two electrodes are required which have different wide operating potential window in which one is in negative range and other in positive to meet resultant higher potential window. In general, the metal oxides are used as cathode, which have a positive working potential window. Frequently, due to negative working potential, carbon electrodes are used as an anode. However, these electrode shows expensive preparation, lower values of Cs and small working potential limits on performance of ASCs device. The materials like copper sulfides (CuS, CuS2 etc.) can operate in wide negative potential window. Therefore, the combination of polymeric gel electrolyte with copper sulfide electrode will be a good contribution in flexible SCs device. On the other side, MnO2 show great attention for positive electrode to fabricate the ASCs device as it shows higher theoretical Cs (1370 F g-1) and highest potential window (> + 0.8 V). Previously, Gao et al.12 fabricated ASCs device based on MnO2/Ni positive electrode, Cao et al.13 assembled ASCs device by MnO2 as a positive and graphene as a negative electrode, Wang et al.14 designed ASCs using MnO2 nanotubes and activated carbon3 ACS Paragon Plus Environment


CNTS, Attias et al.15 selected MnO2 as a positive electrode for ASCs, Fan et al.16 designed graphene/MnO2//ACN ASCs device, Zhang et al.17 synthesized MnO2 composite positive electrode for ASCs and Lei et al.18 grew MnO2 nanofibers on carbon spheres and used as positive electrode in ASCs. In addition, higher conductive and non‒poisonous nature of CuS makes it possible to prepare the negative electrode of ASCs device. Huang et al.19 synthesized CuS/MWCNTs by hydrothermal process and used as electrode, nanostructured CuS networks composed of connected nanoparticles were prepared by Fu et al.20 and used as negative electrode in ASCs, Lu et al.21 used CNT-CuS as an electrode in SCs and Xu et al.22 synthesized CuxS by etching copper foam and studied the electrode properties. Above mentioned earlier reports show limitations on the ED, PD, cycling stability and cost of ASCs device. These drawbacks like lower ED, PD and cycling stability can be eliminated by replacement of carbon based materials with the metal sulfide (CuS) electrodes. This combination of cathode (α-MnO2) and anode (h-CuS) material may enhance the electrochemical performance because of higher conductivity, mechanical stability and wider potential of both electrode materials. There are few metal sulfides such as tin sulfide (SnS)23,24 and molybdenum sulfide (MoS)25 having negative potential window with greater electrochemical performance as compared to carbon based materials but it shows lower potential window than CuS electrode. Comparing with all these negative electrodes, CuS shows wider potential window and higher Cs, ED, PD as well as electrochemical cycling stability. Present work is related to synthesis of α-MnO2 and h-CuS spherical nanostructured electrodes for ASCs device on high mechanical strength, cost efficient and flexible stainless steel (FSS) substrates. Our approach is to fabricate ASCs device for higher energy storage using α4 ACS Paragon Plus Environment

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MnO2/FSS and h-CuS/FSS electrodes using gel electrolyte. After taking preliminary basic characterization of electrodes, also electrochemical properties are tested. Series linked two ASCs devices charged up to 30 s and seven white LEDs are used for performance demonstration of ASCs device. RESULTS AND DISCUSSION Thin film deposition steps and ASCs device fabrication strategies are schematically represented in Scheme 1 (a and b). It shows chemical deposition of α-MnO2 and h-CuS electrodes. The XRD analyses confirm the crystal structure and phase of deposited materials. Figure 1(A) displays XRD of α-MnO2 and h-CuS thin films on FSS substrate. The detected diffraction peaks resemble to (110), (200), (310), (400), (211), (301), (411), (600), (521), (002) and (741) planes, ratify the tetragonal phase of α-MnO2 (JCPDS-44 0141). The low intensity diffraction peaks signify nanocrystalline nature suitable for SCs application.26

Also, XRD

pattern of CuS thin film shows crystalline planes such as (101), (102), (103), (105), (106), (110), (108), (202) and (116) of covellite type h-CuS (JCPDS-00-001-1281, hexagonal structure, a = b = 3.791 and c = 16.4300 Å). The broader and intense peaks in XRD pattern authenticate smaller size of crystallites.27 The peaks shown by asterisk (*) are due to the FSS substrate. Figure 1(B)(a) displays the FTIR spectrum of α-MnO2 thin film. The peak at 3354 cm-1 denotes to – OH stretching vibrations. The peak at the position of 615 cm-1 accompany with the Mn-O- stretching modes.28 Figure 1(B)(b) shows FTIR spectrum of h-CuS film. The peaks seemed at 615, 1020, 1215, 1405, 1628, 2840, 2910 and 3430 cm-1 confirm the formation of h-CuS. The peak observed at 3430 cm-1 is linked toward stretching mode of hydroxyl group. These results for h-CuS film are well matched with literature.29-31 Also, the high intense peaks of both electrode materials 5 ACS Paragon Plus Environment


recommend the availability of more number of functional groups, which can be helpful for SCs. Figure 1(C and D) displays XPS survey spectra of α-MnO2 and h-CuS materials on FSS backbone. The difference of energy between Mn 2p3/2 and Mn 2p1/2 is in well covenant with the literature32-35, suggestive of only α-MnO2 is developed on FSS substrate. The binding energies of 530 and 530.9 eV are assigned to Mn – O – Mn and Mn – O – H, respectively (Figure 1(F)).36 The magnified spectrum of Cu 2p embodied in Figure 1(G), illustrates peaks at 932 and 952.3 eV correlating with Cu2p3/2 and Cu2p1/2, respectively. Furthermore, two small satellite peaks observed at ∼ 934.6 and 954.6 eV correspond to Cu2+ state.37 The binding energies of 162.2 and 169.2 eV related to S 2p3/2 and S 2p1/2 states infer the development of the pure h-CuS phase (Figure 1(H)). These results are in good covenant with the literature values for h-CuS material.3739

Figure 2(A and B) depicts FESEM images of α-MnO2 thin film at 10 KX and 50 KX. The only one nanosphere is focused in Figure 2(B). The diameter of this sphere is about 750 nm. On the outer side of nanosphere, many interconnected nanorods (average size of 65 nm) are observed. The inset shows water contact angle of 29° for α-MnO2 film surface. The transmission electron micrograph of the nanosphere is shown in Figure 2(C). One nanosphere with outer interconnected nanorods is clearly observed in TEM micrograph. Figure 2(D) shows a typical HRTEM image with distinct fringes and an inter-planar spacing of 0.506 and 0.511 nm consistent to (200) and (110) planes, respectively, that agree well with earlier outcomes.40 Inset image shows the SAED pattern of α-MnO2 film. The well observed spots denote (200) and (110) planes. This types of morphology is definitely beneficial for energy storage electrode due to its nanostructure and higher surface area.40-42 Figure 2(E) shows FESEM image at 25 KX magnification. Accordance with this image, elemental mappings of Mn as well as O elements are 6 ACS Paragon Plus Environment

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shown in Figure 2(F and G), respectively. The EDAX analysis spectrum is shown in Figure 2(H). Inset table shows weight and atomic percentage of elements. This analysis confirms the formation of MnO2 thin film on FSS. Figure 3(A and B) displays FESEM images of h-CuS thin film at 10 KX and 25 KX. The shape of h-CuS nanoparticle is exactly not a spherical, but it is a contribution of elliptical as well as spherical nanoparticles. The used precursor and deposition conditions play the important roles in surface modifications. The starting material of CuSO4 (Cu source) and Na2S2O3 (S source) contribute to form nanoparticles. The quick release of S-2 from Na2S2O3 does not control the nanocrystal sizes and only nanospherical particles are developed on surface of FSS substrate. Inset image shows water contact angle of h-CuS thin film indicates hydrophilic nature as it gives 37° angle (inset of Figure 3(A)). Inset of Figure 3(B) shows TEM image of film surface. The well-defined spherical and elliptical shaped nanoparticles of h-CuS materials are seen in TEM analysis (Figure 3(C)). The HRTEM image of h-CuS thin film is depicted in Figure 3(D). The well-defined fringe observed with interplaner distance of 0.33 nm corresponds to (102) plane. Figure 3(E) shows the SAED pattern of h-CuS thin film. The point positions clearly approve the hexagonal phase with (100) and (110) planes. Figure 3(F and G) demonstrates elemental mapping images of Cu and S elements, respectively. It reveals that the uniform distribution of Cu and S elements on film surface. Figure 3(H) depicts the energy dispersive X-ray spectrum for hCuS electrode, where Cu and S peaks are originated. Inset table shows weight (Cu - 40.11%, S 59.89%) and atomic (Cu- 45.06%, S – 54.96%) percentage of Cu and S elements. It discloses that the electrode consists of h-CuS thin film on FSS substrates with well stoichiometry. Therefore, in conclusion, the surface morphologies of α-MnO2 and h-CuS thin film materials are desirable for use in electrode material as charge storage electrodes in supercapacitor devices. 7 ACS Paragon Plus Environment


The type IV adsorption-desorption analysis (Figure S1(A and B)) and corresponding pore size spreading plots (Figure S1(C and D)) of α-MnO2 and h-CuS nanostructures support that the presence of pores on film surface. The BET surface areas of α-MnO2 and h-CuS nanostructures are around 75 and 83 m2 g-1, respectively. For α-MnO2 electrode, pores are distributed on surface and pore radius fluctuating from 5.80 - 25.3 nm. The ranges of pore radii of h-CuS and α-MnO2 material confirm meso/macroporous range. Figure S1(E and F) represents BET plots of α-MnO2 and h-CuS electrode materials. The linearity of the data points designates a strong interaction of α-MnO2 as well as h-CuS electrode material with N2 in the qualified pressure range of 0.0 – 0.3. This is a strong evidence of appropriateness of applying the BET model in defining the surface area. ELECTROCHEMICAL PROPERTIES ELECTROCHEMICAL SUPERCAPACITIVE PROPERTIES OF α-MnO2/FSS ELECTRODE To fabricate efficient ASCs supercapacitor device, choice of the electrode is a most important stage.43-45 Figure 4(A) shows CV curves of α-MnO2 electrode from 5 – 100 mV s-1. Figure 4(B) displays GCD curves of α-MnO2 electrode for 4, 5 and 6 mA cm-2. The reversible redox reaction is as follows,46 + + + + ↔ 1 Further, Cs of electrodes is calculated with respect to scan rates as well as current densities (Figure S2(A and B)). The Cs of 550, 452, 440, 378 and 316 F g-1 are observed for α-MnO2 electrode corresponding to 5 – 100 mV s-1. The approximate values of Cs are detected from charge discharge analysis. The Cs of 505.5, 464, 333 and 200 F g-1 are determined for 4, 5 and 6 8 ACS Paragon Plus Environment

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mA cm-2, respectively. Nanostructure of electrode eases way for interface reactions with electrolyte ions and diffusion resistance is diminished.47 The highest ED and PD are calculated as 61 Wh kg-1 and 2.1 kW kg-1 and are shown in Figure S2(C). The electrochemical stability of α-MnO2 electrode is calculated by repeating CV cycles up to 2000 times at 100 mV s-1. Additionally, the capacity retention with cycle number plot visualizes change in capacity retention with charging discharging process (Figure 4(C)). The CV cycles of electrode are depicted in inset of Figure 4(C). For 2000 CV cycles, α-MnO2 electrode exhibits 92.5 % capacity retention (7.5 % loss of capacitance). Figure 4(D) shows Nyquist plot of electrode in 1 M Na2SO4 aqueous electrolyte (Inset displays fitted equivalent circuit). The charge transfer (Rct) and equivalent series (ESR) resistances of 2.44 and 2.91 Ω cm-2 are observed. Overall observed electrochemical parameters (Cs, ED, PD and cycling stability) of αMnO2 electrode endorse auspicious electrode material for ASCs device48. ELECTROCHEMICAL SUPERCAPACITIVE PROPERTIES OF h-CuS/FSS ELECTRODE The CV curves are recorded from potential of - 1.0 to + 0.2 V/SCE (Figure 5(A)). The intercalation/deintercalation reaction of electrode electrolyte is as shown below,49,50 + ↔ + 2 The charge discharge curves for h-CuS electrode attained in a potential range between -1.0 to + 0.2 V/SCE at 1, 4, 5 and 6 mA cm-2 and represented in Figure S3(A). In charging profiles, the position ‘O’ indicates oxidation and position ‘R’ shows reduction correspondence as observed in CV curves of electrode. The non-linear region in the charging curve is the evidence that h-CuS electrode shows redox reactions. Figure 5(B) shows plot of Cs versus scan rate. The Cs of 885 F 9 ACS Paragon Plus Environment


g-1 is calculated at 5 mV s-1 for h-CuS electrode. The Cs of 690 F g-1 is observed for h-CuS electrode at current density of 4 mA cm-2 (Figure S3(B)). Furthermore, the values of ED and PD are calculated from formulae reported in literature.51,52 The ED and PD of 62 Wh kg-1 and 1.7 kW kg-1, respectively are reported for h-CuS electrode and included in Ragone plot (Figure S3(C)). The capacity retention versus CV cycle plot is depicted in Figure 5(C). The CV cycling stability of h-CuS electrode is calculated by repeating CV curves at 100 mV s-1 for 2000 times (Inset of Figure 5(C)). The Cs of h-CuS electrode is 885 F g-1 for the second cycle, as cycle number increases, Cs of 812 F g-1 is observed for 2000th cycle, denoting cycling stability of 92 % (8 % capacitance loss). Moreover, the conductibility of the prepared h-CuS electrode is premeditated by EIS measurement and included in the Nyquist plot (Figure 5(D)). The EIS measurements are done at potential of 10 mV and frequency ranging from 0.1 Hz to 1 MHz. The equivalent circuit of Nyquist plot is shown in inset. Plot shows Rs, which is developed due to the internal resistance of interface, ionic resistance of electrolyte and the internal resistance of electrode.53 The Rct of electrode material is determined by taking diameter of semicircle at high frequency on Nyquist plot. The straight plot after semicircle indicates Warburg impendence (W) connected to the ion diffusion resistance while intercept on x-axis stretches Rs.54 For h-CuS electrode, the Rs of 0.9 Ω cm-2 and Rct of 3.96 are detected in 2 M KOH electrolyte, that represent improved electronic conductivity and electro-activity of h-CuS electrode. The achieved higher performance of h-CuS electrode owing to decent conductivity of FSS substrate (decreasing interfacial resistance) and nanostructure of surface of electrode (enhancing contribution of electro-active material in electrochemical reactions). The nanoparticles of h-CuS surface access the electrolyte ions and gives small way for intercalation reaction by dropping 10 ACS Paragon Plus Environment

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resistances. Complete electrochemical performance shows ability of h-CuS electrode as a negative electrode in fabrication of ASCs supercapacitor. The complete charge storing kinetics of positive and negative electrodes are investigated using the relation of current (I) and scan rate observed in CV curves, as following equation: I = aVb

(3)

where ‘I’ is the highest current (mA), ‘a’ and ‘b’ are coefficients and ‘V’ is scan rate (mV s-1). The type of charge storing mechanism is described by ‘b’ values, either it is capacitive or diffusion controlled. Although, if b = 1 then it suggests the contribution comes from capacitive behavior and b = 0.5 suggests the semi-infinite diffusion-controlled process. The ‘b’ values of 0.83 and 0.77 and ‘R2’ values of 0.9876 and 0.9880 are observed for α-MnO2 and h-CuS electrodes, respectively (Figure 6(A)). Figure 6(B and C) specifies highest current with the square root ‘V’ for α-MnO2 and h-CuS electrodes. Ipo and Ipr denote the anodic and cathodic current densities. The peak current densities of α-MnO2 (5.40 and 6 mA cm-2) and h-CuS (38.3 and 36.5 mA cm-2) are proportional to the square root of ‘V’. The oxidation and reduction currents varying with ‘V’ signify the electrochemical ideal capacitive behavior of α-MnO2 and hCuS electrodes. ELECTROCHEMICAL

SUPERCAPACITIVE

PROPERTIES

OF

α-MnO2/FSS/PVA-

Na2SO4/α-MnO2/FSS SYMMETRIC DEVICE The symmetric α-MnO2/FSS/PVA-Na2SO4/α-MnO2/FSS device is made-up by PVANa2SO4 gel electrolyte and α-MnO2/FSS electrodes. The appropriate operating potential window of symmetric device is selected by taking CV curves at different potentials (+ 0.8 ̶ + 1.4 V) (Figure S4(A)). This potential is also confirmed by GCD analysis. The GCD curves are taken at 11 ACS Paragon Plus Environment


different potentials of + 0.6 ̶ + 1.2 V at constant current of 3 mA (Figure S4(B)). The maximum potential of + 1.2 V is achieved by symmetric supercapacitor device. The charge discharge curves at 3 ̶ 5 mA are depicted in Figure S4(C). The CV of symmetric device at 5 ̶ 100 mV s-1 show higher area under CV curve for 100 mV s-1 (Figure S4(D)). As more electro-active electrode material contributes in electrochemical reaction at lower scan rate/current density due to availability of time, it gives higher Cs as comparing with higher scan rate/current density (Figure S4(E and F)). The Cs of 67.3 and 54.6 F g-1 are calculated for α-MnO2/FSS/PVANa2SO4/α-MnO2/FSS symmetric device. The flexibility of device is tested by measuring CV at different bending positions of device (Figure S5(A)). At 180° bending position, device losses its Cs up to 63 F g-1 (Figure S5(B)). The Nyquist plot analysis carried out at 10 mV. The Rs of 0.6 Ω cm-2 and Rct of 7.4 Ω cm-2 are observed for symmetric device (Figure S5(C)). The capacity retention with respect to cycle number plot is signified in Figure S5(D). Furthermore, CV cycles are taken up to 1500 cycles to test the stbility of symmetric device (Inset of Figure S5(D)). The maximum capacity loss of 12.8 % (87.2 % stability) is observed after 1500 CV cycles for FSS symmetric device. The Bode plot analysis and real and imaginary capacitance disparity with applied frequency is depicted in the Figure S5(E and F). Inset photographs indicate demonstration of symmetric device by lightning one LED for the periods of three min after 30 s charging. ELECTROCHEMICAL SUPERCAPACITIVE PROPERTIES OF h-CuS/FSS/PVA-KOH/hCuS/FSS SYMMETRIC DEVICE The electrochemical properties of h-CuS/FSS/PVA-KOH/h-CuS/FSS symmetric device are tested to confirm the best design of SCs for higher energy storage. The potential window is 12 ACS Paragon Plus Environment

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selected by taking CV curves from + 0.6 to + 1.4 V (Figure S6(A)). Furthermore, GCD curves of symmetric device are carried out at current of 3 ̶ 5 mA (Figure S6(B). The CV curves at 5 – 100 mV s-1 are taken at selected + 1.2 V potential (Figure S6(C)). The Cs of 60.1 F g-1 is observed at 5 mV s-1 (Figure S6 (D)) and 43 F g-1 is calculated at 3 mA (Figure S6(E)). Inset photograph shows demonstration of symmetric device by glowing two yellow and two red LEDs up to two min. Figure S6(F) shows capacity retention with cycle plot and inset depicts CV curves at different cycle numbers for 2000 cycles. Device shows 84.3 % capacity retention for 2000 cycles. Figure S7(A) shows the Bode plot of symmetric device. The phase angle of 24.90° is observed for h-CuS/FSS/PVA-KOH/h-CuS/FSS symmetric device. The dissimilarity of capacitance with frequency plots is analyzed using impedance measurements (Figure S7(B)). The Rs and Rct of 1.93 and 3.17 Ω cm-2 are observed for symmetric device (Figure S7(C)). Flexibility of device is tested by taking CV cycles at bending position of 0, 90 and 180° (Figure S7(D)). The performance of α-MnO2/FSS/PVA-Na2SO4/α-MnO2/FSS and h-CuS/FSS/PVAKOH/h-CuS/FSS symmetric devices are not sufficient to use these devices at commercial level. So, the strategy is to enhance the performance of solid state device by designing asymmetric supercapacitor. Asymmetric design shows advantages over symmetric one as it provides wide potential window, cycling stability and ED. The fabrication steps of α-MnO2/FSS/PVANa2SO4/α-MnO2/FSS and h-CuS/FSS/PVA-KOH/h-CuS/FSS symmetric devices as well as αMnO2/FSS /PVA-LiClO4/h-CuS/FSS ASCs asymmetric device are mentioned in Scheme 2 (A ̶ D). The CV cycles at 100 mV s-1 for above devices are included in Scheme 2 (E ̶ G). The size difference (area under CV curves) are analyzed by plotting all CV curves in one plot (Scheme 2



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(H)). The symmetric devices achieves operating potential up to + 1.2 V and asymmetric device goes up to + 1.8 V potential for single device. ELECTROCHEMICAL

SUPERCAPACITIVE

PROPERTIES

OF

α-MnO2/FSS

/PVA-

LiClO4/h-CuS/FSS ASCs DEVICE The ASCs device is fabricated using α-MnO2/FSS and h-CuS/FSS electrodes and the individual CV plots of these electrodes are depicted in Figure 7(A). Before fabrication of ASCs device, the mass ratio of electrodes were adjusted as 0.46 according with equation 5. Figure 7(B) depicts CV curves of ASCs device for different voltage ranges varying from + 1.0 to + 2.0 V at 100 mV s-1. It shows that the current response and the area under CV curve increases up to + 1.8 V indicating + 1.8 V potential is a suitable potential for ASCs device. The nature of CV curves denotes the pseudocapacitive faradaic reactions of α-MnO2 and h-CuS.55,56 The CV curves of ASCs device at 5 to 100 mV s-1 are displayed in Figure 7(C). The CV curves of α-MnO2 and hCuS electrodes and ASCs device are plotted in Figure S8(A). As compared with these two electrodes in aqueous electrolyte, the performance of ASCs device using polymer gel electrolyte is high. The Cs of 109.1 F g-1 is observed at 5 mV s-1 (Figure S8(B)). The available time at 5 mV s-1 is more than 100 mV s-1, hence the more number of electro active material exists at lower scan rate, resulting as higher Cs. The GCD curves of α-MnO2/FSS, h-CuS/FSS electrodes and α-MnO2/FSS/PVALiClO4/h-CuS/FSS ASCs supercapacitor devices are plotted in one graph (Figure 7(D)). The columbic efficiency of ASCs device at + 1.8 V potential is high (Figure 7(E)). It is observed that the potential of + 1.8 V is suitable for ASCs device. This analysis reveals that the highest performance of ASCs device is achieved by using combination of α-MnO2 and h-CuS electrodes. 14 ACS Paragon Plus Environment

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The GCD curves of ASCs device at currents of 4, 6, 8 and 10 mA are shown in Figure 7(F). The highest Cs of 85.6 F g-1 is attained at current of 4 mA (Figure S8(C)). The bending positions of device at 0, 45, 90, 135 and 180° of the ASCs are shown in Figure 7(G ̶ K). The obtained capacitance at different bending positions is plotted in Figure S7(D). The negligible change in capacitance (overlapping of CV cycles) is observed for ASCs device at different bending positions of device (inset of Figure S8(D)). The overlapping of CV curves on each other indicates negligible effect of bending of device on area under CV curve. The Cs at bending of 180° is retained up to 98 % (only 2 % loss). The GCD cycles for h-CuS/FSS/PVA-KOH/h-CuS/FSS, α-MnO2/FSS/PVA-LiClO4/αMnO2/FSS symmetric and α-MnO2/FSS/PVA-LiClO4/h-CuS/FSS ASCs devices are plotted in Figure 8(A). Figure 8(B) illustrates the Nyquist plot of ASCs device. The Rct of 2.2 Ω and Rs of 1.2 Ω are seen for ASCs. The electrochemical cycling stability of ASCs supercapacitor device examined by taking CV curves up to 5000 cycles (Figure 8(C)). To visualize the cycling stability difference of symmetric and asymmetric SCs, the comparative capacity retention is plotted in Figure 8(D). The highest stability of 93.3% is observed for ASCs device as compared to αMnO2/FSS/PVA-Na2SO4/α-MnO2/FSS (87.2%, 1500 CV cycles) and h-CuS/FSS/PVA-KOH/hCuS/FSS (84.3%, 2000 CV cycles). The achieved better electrochemical stability owing to electrode design, performance of electrode materials and used polymer gel electrolyte. Inset photographs show demonstration of symmetric as well as asymmetric SCs devices by glowing different types of LEDs. Bode plot of ASCs is depicted in Figure 8(E). The shape and phase angle confirm capacitive behavior of ASCs device.



The demonstration of fabricated ASCs device is carried out with table lamp and two ASCs devices. After 30 s charge, during discharging of ASCs device, seven white LEDs glow for more than seven min. Figure 9(A and B) displays photographs captured at discharging of ASCs device. Figure 9(C) exhibits photograph of table lamp after seven minutes. The ED and PD values of ASCs device calculated using equation 6 and 7 are plotted in Figure 9(D). The ED and PD of 18.9 Wh kg-1 and 32 kW kg-1 are deliberated for ASCs device. The Ragone plot obtained from symmetric and ASCs devices are compared and plotted in one plot (Figure S9). The electrochemical parameters of α-MnO2, h-CuS, symmetric and asymmetric devices are included in Table 1. Furthermore, the obtained electrochemical supercapacitive properties of ASCs device compared with previous work on MnO2 based ASCs are included in Table 2. All electrochemical parameters of ASCs shows higher values as compared with symmetric one. CONCLUSIONS In this work, for the first time, an asymmetric supercapacitor fabricated using the positive α-MnO2/FSS and negative h-CuS/FSS electrodes and a polymer gel electrolyte (PVA-LiClO4). The nanospheres like surface morphology of α-MnO2 and h-CuS thin films achieved from simple, binderless and scalable chemical bath deposition method displays excellent electrochemical properties. The 3D porous nanospheres of α-MnO2 and h-CuS serve as an outstanding three-dimensional electrodes. The potential difference of positive and negative electrodes allows + 1.8 V potential window for ASCs device compared with symmetric devices. The assembled α-MnO2/FSS/PVA-LiClO4/h-CuS/FSS ASCs supercapacitor device exhibits higher electrochemical features such as Cs (109.12 F g-1), ED (18.9 Wh kg-1), PD (32 kW kg-1) and electrochemical stability (93.3 % for 5000 CV cycles). The first time use of α-MnO2 and h16 ACS Paragon Plus Environment

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CuS electrodes for ASCs device demonstrates (seven white LEDs for more than seven minutes after 30 s charging) the new approach to fascinate energy storing ability of ASCs device. EXPERIMENTAL DETAILS SYNTHESIS OF α-MnO2 NANOSPHERES (NSS) Initially, in the beakers 2 ml of methanol was added in 0.1 M KMnO4 solution and FSS substrates were immersed. These baths were placed at room temperature for 8, 12, 16 h. The blackish brown colored α-MnO2 thin films were developed on substrates. The thickness of thin film gradually increases up to 12 h, after that the overgrowth of material is observed. The thickness of prepared thin films observed to be 406, 586 and 576 nm, respectively. The mass loading on substrate is about 0.23, 0.34 and 0.28 mg (1 cm2 area) for 8, 12, 16 h time of deposition, respectively. Surface morphologies of thin films deposited at different deposition time is included in Figure S10(A ̶ C), respectively. Well adherent thin film with maximum thickness and mass is observed for 12 h time. After 12 h, due to overgrowth of electrode material, the drop of material from substrate is observed (Figure S10(D)). Hence, α-MnO2 thin film deposited at 12 h is continued for further electrochemical investigations. SYNTHESIS OF h-CuS NSs The 0.1 M CuSO4 solutions were prepared in three separate beakers containing 50 ml double distilled water (DDW) and then 0.8 ml of triethylamine (TEA) and 0.15 M of Na2S2O3 solutions was added in each beaker (pH ~ 2.5 (± 0.1)). Deposition of films were carried out at 333, 343 and 353 K for 3 h. Surface morphologies of these three different films are contained within in Figure S11(A-C), respectively. The rate of chemical reaction alters the surface structure and thickness of thin film. At 343 K, the highest thickness of film is observed to be 785 nm as 17 ACS Paragon Plus Environment


compared to 333 K (602 nm) and 353 K (696 nm). The mass loading on FSS was optimized by changing reaction bath temperature. The optimized mass loaded on FSS is 0.405 mg for the area of 1 cm2. The observed mass loading for different thin films deposited at 333, 343 and 353 K is depicted in Figure S11(D). ASSEMBLY OF α-MnO2/FSS/PVA–Na2SO4/α-MnO2/FSS, h-CuS/FSS/PVA-KOH/h-CuS/FSS AND α-MnO2/FSS/PVA-LiClO4/h-CuS/FSS ASCs DEVICES The symmetric SCs device designed by two α-MnO2/FSS electrodes and PVA-Na2SO4 solid gel electrolyte (4 g of PVA and 0.852 g of Na2SO4). Similarly, h-CuS/FSS/PVA-KOH/hCuS/FSS device fabricated using two h-CuS/FSS electrodes and PVA-KOH gel electrolyte. The PVA-KOH gel electrolyte (4.5 g of PVA and 0.337 g of KOH). After packing of symmetric devices using plastic strips, these devices placed in hydraulic press for 24 h at 1 ton pressure. The α-MnO2/FSS/PVA-LiClO4/h-CuS/FSS ASCs device was assembled using α-MnO2, h-CuS electrodes and PVA-LiClO4 gel electrolyte (6 g of PVA and 1 M LiClO4). The thickness of 450 µm is maintained due to adjacent contact between the electrolyte and electrodes. CHARACTERIZATION OF α-MnO2 AND h-CuS ELECTRODES: The basic characterizations were carried out by X-ray diffraction (XRD) technique (AXS D8), fourier transform infrared (FT-IR) spectroscopy, X-ray photoelectron spectroscopy (XPS) (VG Multilab 2000), Brunauer, Emmett and Teller (BET) and Barrett–Joyner–Halenda (BJH) (Quantachrome v11.02), field emission scanning electron microscopy (FESEM) and elemental analyses (EDAX)(JEOL JSM 6390), contact angle (Rame Hart instrument) and high-resolution transmission electron microscopy (HRTEM) analysis (JEOL-3010). The electrochemical properties were tested using automatic battery cycler (WBCS-3000 and ZIVE-MP1). 18 ACS Paragon Plus Environment

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The Cs of electrode materials and ASCs device were calculated using following equation

%$1 Cs = ! I V dV 4 MS V − V $&

Where, ‘M’ is mass of material, ‘S’ is scan rate, (V1 – V0) is potential and ‘I ‘is current density. Furthermore, the electrochemical properties of symmetric and ASCs devices were measured using two electrode system. The ratio of masses of α-MnO2/FSS and h-CuS/FSS electrode materials adjusted using following equation, M. i Cs. i × V. i = 5 M. ii Cs. ii × V. ii Where, M.i and M.ii, Cs.i and Cs.ii and V.i and V.ii are the masses, Cs and potential of α-MnO2 and h-CuS electrodes, respectively. The ED and PD of electrode materials as well as devices were calculated by following equations, ED = PD =

0.5 × Cs × V − V , and 6 3.6

3600 × ED 7 dt

Where, V1 - V0 is potential window. ASSOCIATED CONTENT SUPPORTING INFORMATION Supporting Information contains BET study, Cs versus scan rate and current density plots of αMnO2/FSS and h-CuS/FSS electrodes, CV curves, capacitance plots, electrochemical properties 19 ACS Paragon Plus Environment


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at different bending angles for α-MnO2/FSS/PVA-Na2SO4/α-MnO2/FSS symmetric device, electrochemical

properties

of

h-CuS/FSS/PVA-KOH/h-CuS/FSS

symmetric

device,

electrochemical properties of α-MnO2/FSS/PVA-LiClO4/h-CuS/FSS ASCs device and Ragone plots for symmetric and asymmetric devices, FESEM images and thickness, mass loading versus deposition time plots of α-MnO2 thin films and FESEM images and thickness, mass loading versus deposition temperature plots of h-CuS thin films. AUTHOR INFORMATION Corresponding Author Prof. Chandrakant D. Lokhande Tel: + 91-231-2601212 E-mail:- [email protected] ACKNOWLEDGEMENT Financial assistance from Department of Science and Technology Science and Engineering Research Board (DST-SERB), New Delhi, India (SERB/F/7448/2016-17) is acknowledged. REFERENCE 1. Zhang, L.; Zhao, X.; Carbon-Based Materials as Supercapacitor Electrodes. Chem. Soc. Rev. 2009, 38, 2520-2531. 2. Simon, P.; Gogotsi, Y. Materials for Electrochemical Capacitors. Nat. Mater. 2008, 7, 845854.


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60. Chen, J.; Wang, Y.; Cao, J.; Liu, Y.; Ouyang, J.; Ji, D.; Zhou, Y. Flexible and Solid-State Asymmetric Supercapacitor Based on Ternary Graphene/MnO2/Carbon Black Hybrid Film with High Power Performance. Electrochim Acta. 2015, 182, 861-870. 61. Lian, L.; Yang, J.; Xiong, P.; Zhang, W.; We, M. Facile Synthesis of Hierarchical MnO2 Sub-Microspheres Composed of Nanosheets and Their Application for Supercapacitors. RSC Adv. 2014, 4, 40753-40757. 62. Le, Q.; Wang, T.; Tran, D.; Dong, F.; Zhang, Y.; Losic, D. Morphology-Controlled MnO2 Modified Silicon Diatoms for High-Performance Asymmetric Supercapacitors. J Mater. Chem. A. 2017, 5, 10856-10865. 63. Khomenko, V.; Raymundo E. P.; Frackowiak, E.; Beguin, F. High-Voltage Asymmetric Supercapacitors Operating in Aqueous Electrolyte. Appl. Phys. A: Mater. Sci. Process. 2006, 82, 567-573. 64. Liu, W.; Li, X.; Zhu, M.; He, X. High-Performance All-Solid State Asymmetric Supercapacitor Based on Co3O4 Nanowires and Carbon Aerogel. J Power Source. 2015, 282, 179-186. 65. Cottineau, T.; Toupin, M.; Delahaye, T.; Brousse, T.; Belanger, D. Nanostructured Transition Metal Oxides for Aqueous Hybrid Electrochemical Supercapacitors. Appl. Phys. A: Mater. Sci. Process. 2016, 82, 599-606. 66. Zhang, X.; Yu, P.; Zhang, H.; Zhang, D.; Sun, X.; Ma, Y. Rapid Hydrothermal Synthesis of Hierarchical Nanostructures Assembled from Ultrathin Birnessite-Type MnO2 Nanosheets for Supercapacitor Applications. Electrochim. Acta. 2013, 89, 523-529. 29 ACS Paragon Plus Environment


67. Wang, H.; Holt, C.; Li, Z.; Tan, X.; Amirkhiz, B. S.; Xu, Z.; Olsen, B. C.; Stephenson, T.; Mitlin, D. Graphene-Nickel Cobaltite Nanocomposite Asymmetrical Supercapacitor with Commercial Level Mass Loading. Nano Res. 2012, 5(9), 605-617. 68. Zilong, W.; Zhu, Z.; Qiu, J.; Yang, S. High Performance Flexible Solid-State Asymmetric Supercapacitors from MnO2/ZnO Core–Shell Nanorods//Specially Reduced Graphene Oxide. J Mater. Chem. C. 2014, 2, 1331-1336. 69. He, Y.; Chen, W.; Li, X.; Zhang, Z.; Fu, J.; Zhao, C.; Xie, E. Freestanding ThreeDimensional Graphene/MnO2 Composite Networks As Ultralight and Flexible Supercapacitor Electrodes. ACS Nano. 2013, 7, 174-182. 70. Radhamani, A.; Shareef, K.; Rao, M. ZnO@MnO2 Core–Shell Nanofiber Cathodes for High Performance Asymmetric Supercapacitors. ACS Appl. Mater. Interfaces. 2016, 8(44), 3053130542.


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Scheme 1 (a) Schematic diagram illustrating synthesis procedure of α-MnO2 and h-CuS on flexible stainless steel (FSS) substrates, and (b) α-MnO2/FSS/PVA-LiClO4/h-CuS/FSS asymmetric supercapacitor device fabrication steps.



Figure 1 (A) XRD patterns and (B) FT-IR spectra of (a) α-MnO2 and (b) h-CuS thin films on FSS substrates, (C) XPS survey spectrum of α-MnO2 thin film, (D) XPS survey spectrum of hCuS thin film, (E) Mn 2p spectrum, (F) O 1s spectrum, (G) Cu 2p spectrum, and (H) S 2p spectrum. 32 ACS Paragon Plus Environment

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Figure 2 (A and B) FESEM images of α-MnO2 thin film at magnifications of 10 KX and 50 KX (inset shows water contact angle), (C) TEM image of α-MnO2 powder sample, (D) HRTEM image of α-MnO2 sample (inset shows SAED image of α-MnO2), (E) FESEM image of α-MnO2 thin film at magnification of 25 KX, (F) elemental percentage of Mn and (G) elemental percentage of present in α-MnO2 thin film, and (H) EDAX spectrum of α-MnO2 thin film (inset table shows atomic and weight percentage of Mn and O elements). 33 ACS Paragon Plus Environment


Figure 3 (A and B) FESEM images of h-CuS thin film at magnifications of 10 KX and 50 KX (inset shows water contact angle and magnified TEM image), (C) TEM image, (D) HRTEM image of h-CuS electrode, (E) SAED image of h-CuS, (F and G) elemental mapping images of Cu and S elements, and (H) EDAX spectrum of h-CuS (inset shows weight and atomic percentage of Cu and S elements).


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Figure 4 (A and B) CV curves at different scan rates (5 – 100 mV s-1) and GCD curves at various current densities (4, 5 and 6 mA cm-2) of α-MnO2 electrode, (C) capacity retention with cycle number plot (inset shows CV curves at scan rate of 100 mV s-1 for different cycles), and (D) Nyquist plot of α-MnO2 electrode (inset shows equivalent circuit from which Nyquist plot is generated).



Figure 5 (A) CV curves at different scan rates (5 – 100 mV s-1), (B) Cs versus scan rate plot, (C) capacity retention versus cycle number plot (inset shows CV cycling curves up to 2000 cycles at 100 mV s-1), and (D) Nyquist plot of h-CuS electrode (inset shows equivalent circuit).


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Figure 6 (A) Plots of Log(anodic current density) against Log(scan rate) for α-MnO2 and h-CuS in order to calculate the ‘b’ values in the CV curves from 5 to 100 mV s-1, and (B and C) the plots of oxidation and reduction peak current versus the square root of scan rate for α-MnO2 and h-CuS electrodes. 37 ACS Paragon Plus Environment


Scheme 2 Schematic Representation of Fabrication Steps for (A) α-MnO2/FSS/PVA-Na2SO4/αMnO2/FSS, (B) h-CuS/FSS/PVA-KOH/h-CuS/FSS and (C and D) α-MnO2/FSS/PVA-LiClO4/hCuS/FSS ASCs Device Fabricated by α-MnO2 and h-CuS Electrode using PVA-LiClO4 Gel Electrolyte, CV Curves at 100 mV s-1 for (E) α-MnO2/FSS/PVA-Na2SO4/α-MnO2/FSS (Inset shows Photograph of Demonstration by Glowing Red LED), (F) h-CuS/FSS/PVA-KOH/hCuS/FSS (Inset shows Photograph of Demonstration by Glowing Red LED), (G) αMnO2/FSS/PVA-LiClO4/h-CuS/FSS ASCs (Inset shows Photograph of Demonstration by Glowing White LEDs), and (H) Comparative CV Curves in One Plot.


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Figure 7 (A) CV curves of α-MnO2 and h-CuS electrodes at scan rate of 100 mV s-1, (B) potential windows selection CV plots at different potential ranging from + 1.0 to + 2.0 V, (C) CV curves at different scan rates (5–100 mV s-1) α-MnO2/FSS/PVA-LiClO4/h-CuS/FSS ASCs device, (D) GCD curves of α-MnO2, h-CuS electrodes and ASCs device at 4 mA, (E) The GCD curves of ASCs device at 4 mA for potentials of +1.0-1.8 V, and (F) GCD curves of ASCs device at 4 ̶ 10 mA, (G-K) different bending positions of ASCs device from 0-180°.



Figure 8 (A) GCD curves of α-MnO2/FSS/PVA-Na2SO4/α-MnO2/FSS, h-CuS/FSS/PVAKOH/h-CuS/FSS and α-MnO2/FSS/PVA-LiClO4/h-CuS/FSS ASCs devices at 4 mA, (B) Nyquist plot of ASCs, (C) CV curves at 2nd ̶ 5000th cycles for ASCs device, (D) capacity retention versus cycle number plots for α-MnO2/FSS/PVA-Na2SO4/α-MnO2/FSS, h-CuS/FSS/PVA-KOH/hCuS/FSS symmetric and ASCs device up to 1500th, 2000th and 5000th CV cycles, and (E) Bode plot of ASCs device.


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Figure 9 (A and B) Demonstration photographs at initial and (C) after seven minutes for αMnO2/FSS/PVA-LiClO4/h-CuS/FSS ASCs device using seven white LEDs of table lamp, and (D) Ragone plot of ASCs device (inset shows performance comparison with literature, FESEM images of (a) α-MnO2, (b) h-CuS and (c) demonstration photograph with table lamp).



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Table 1. The calculated electrochemical parameters of α-MnO2, h-CuS, symmetric and asymmetric supercapacitor devices. Sr.

Sample

Electrolyte Specific

No

1

α-MnO2

1M

Energy

Power

Capacity

Capacitan

density

density

retention

-ce (F g-1)

-1

kW kg-1

(%)

(W kg )

Rct

ESR

Ωcm-2

Ωcm-2

550

61

2.1

92.5

2.91

2.44

2 M KOH

885

62

1.7

92

3.96

0.9

PVA-

67.3

15.3

13

87.2

7.4

0.6

60.1

12.5

16

84.3

3.17

1.93

109.1

18.9

32

93.3

2.2

1.2

Na2SO4 2

h-CuS

3

α-MnO2/FSS/PVANa2SO4/α-

Na2SO4

MnO2/FSS 4

5

h-CuS/FSS/PVA-

PVA-

KOH/h-CuS/FSS

KOH

α-MnO2/FSS/PVA-

PVA-

LiClO4/h-CuS/FSS

LiClO4


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Table 2. Comparative chart for electrochemical performance of ASCs devices reported in literature with present study. Sr. No

Supercapacitor device

Electrolyt e

1

Graphene(ILCMG)//RuO 2–IL-CMG

H2SO4/PV A gel

2

MnO2//Fe3O4

0.1 M K2SO4

3

CoMoO4/MnO2 FeWO4/MnO2

4

Power Specific capacitance density (kW kg-1) (Cs)

Energy density (Wh kg-1)

Electrochemical stability (%)

Ref.

175 F g-1 at 6.8

19.7

20 F g-1

0.8

7

5,000 cycles

KOH

152 F g-1

0.8

54

84 after 10,000 8 at 3 A g-1

5M LiNO3

-

-

-

Reported for 9 40,000 cycles

0.5 A g-1

̶

5 GCD

7

5

NiO//α-Fe2O3

PVA/KO H

57.2

0.951

12.4

85 after 10,000 10 cycles

6

NiO//NiCo2O4

6 M KOH

89

2.5

25.99

80.2 after 2000 11 cycles

7

GH//MnO2-NF

Na2SO4

41.7

10

14.9


147

0.1

25.2

96 after cycles

500

-

0.1

24.8

92 after cycles

6000

0.338

7.8

85 after cycles

7000

22.1


1 mol L−1 8

MnO2/graphene Na2SO4

9

MnO2 NT//AC–CNTs

2.0 M Li2SO4

10

MnO2//AC

0.5 M 137 K2SO4

11

12

CuO@MnO2//MEGO

CoSe2//MnO2

49.2 F g-1 at 0.25 A g- 85.6

Na2SO4

1

LiCl/PVA gel

1.77 F cm-3 0.282 at 1 mA Wcm-3 cm-2 43

ACS Paragon Plus Environment

13

14 15

0.58 mWh 94.8 after 2000 58 cm-3 cycles


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13

CNTs@NCS@MnO2//AC

Na2SO4

312.5

220

27.3

92.7 after 4000

59

14

rGO/MnO2/CB//rGO/CB

Na2SO4

209

21

24.3

89 after 1000

60

15

MnO2//AC

Na2SO4

0.885

15.84

88 after 1000

61

16

Sidiatom@MnO2//AGO

PVDFNa2SO4

65.1 F g-1 2.22 at 0.5 A g-1

23.2

84.8 after 2000

62

17

MnO2//PEDOT

KNO3

-

120

13.5

-

63

18

MnO2//PPy

KNO3

-

62.8

7.37

-

63

19

PANI//Carbon Maxsorb

KNO3

-

45.6

11.46

-

63

20

PEDOT//Carbon Maxsorb

KNO3

-

54.1

3.82

-

63

21

MnO2//PANI

KNO3

-

42.1

5.86

-

63

22

PPy//Carbon Maxsorb

KNO3

-

48.3

7.64

-

63

23

Carbon aerogel//Co3O4

KOHPVA gel

0.750

17.9

85 after cycles

24

MnO2//Fe3O4

K2SO4

21.5

10.2

8.1

-

65

25

MnO2//Carbon

K2SO4

31

19

17.3

-

65

26

MnO2//AC

Na2SO4

269

0.1

17.1

94 after 2000

66

27

MnO2//AC

K2SO4

22.9

16

10

-

66

28

GNCC//AC

KOH

288

5.6

19.5

102 after 10000

67

LiCl/PVA gel

0.52 F cm-3 0.133 at 10 mV s1 Wcm-3

0.23 mWh cm


Na2SO4/P VP gel ̶

1.20

24.8

-

ZnAcPVA

33

6.5

17

97 after cycles

29

ZnO@MnO2//RGO CNTs/MnO2//CNTs/

30 PANI 31

ZnO@MnO2//AC

57.4 F g-1 1 Ag-1


-3

1000

64

69 5000

70

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32

α-MnO2/FSS/PVALiClO4/h-CuS/FSS


PVALiClO4

109 at 5 32 mV s-1


18.9

This 93.3 after 5000 Wor cycles k


For TABLE OF CONTENT (TOC) only


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Flexible Asymmetric Solid-State Supercapacitors by Highly Efficient

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