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Cite This: ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Flexible Asymmetric Solid-State Supercapacitors by Highly Efficient 3D Nanostructured α‑MnO2 and h‑CuS Electrodes Amar M. Patil,† Abhishek C. Lokhande,‡ Pragati A. Shinde,† and Chandrakant D. Lokhande*,† †

Centre of Interdisciplinary Research, D. Y. Patil University, Kolhapur 416006, Maharashtra, India Department of Materials Science and Engineering, Chonnam National University, Gwangju 500-757, South Korea

‡

S Supporting Information *

ABSTRACT: A 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/flexible stainless steel (FSS)/polyvinyl alcohol (PVA)−LiClO4/h-CuS/FSS asymmetric supercapacitor (ASC) devices have been fabricated using PVA−LiClO4 gel electrolyte. Highly active surface areas of α-MnO2 (75 m2 g−1) and h-CuS (83 m2 g−1) electrodes contribute to more electrochemical reactions at the electrode and electrolyte interface. The ASC 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 a capacity retention of 93.3% after 5000 cycles. Additionally, ASC devices were successful in glowing seven white-light-emitting diodes for more than 7 min after 30 s of charging. Outstandingly, real practical demonstration suggests “ready-to-sell” products for industries. KEYWORDS: asymmetric supercapacitor, electrochemical cycling stability, electrodes, nanospheres, h-CuS, α-MnO2

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INTRODUCTION Portable and small supercapacitors (SCs) are essential in the assembly of recent energy storage devices,1,2 as they show mechanical flexibility and are lightweight, low-cost, and environment-friendly energy storage devices, which have applications in portable, wearable, and commercialized pocket electronic devices.3 There is a need to upsurge the energy (ED) as well as power (PD) of energy storage devices using different electrode materials and electrolytes, which can run at a wide potential range to tune the working potential of these energy storage devices. The asymmetric design of supercapacitors is a practical way to augment the potential of these devices. As an impact of the potential window of asymmetric supercapacitors (ASCs), they show higher ED, PD, and specific capacitance (Cs) compared to those of the corresponding symmetric supercapacitors.4−6 Various combinations of anode and cathode have been reported in the literature, such as MnO2//Fe3O4,7 CoMoO4/MnO2,8 FeWO4/MnO2,9 NiO//α-Fe2O3,10 NiO//NiCo2O4,11 and so on. Solid electrolyte-based SCs possess many advantages, such as small size, lightweight, exceptional reliability, and a higher operating temperature. Polymer-based gel electrolytes offer a polymeric chain to carry out the redox reaction between the electrolyte and electrode interfaces. For the fabrication of ASC devices, two electrodes are required, which have different wide operating potential windows, in which one is in the negative range and the other in positive to meet the resultant higher potential window. In general, metal oxides are used as the cathode, which have a positive working © XXXX American Chemical Society

potential window. Frequently, due to negative working potential, carbon electrodes are used as an anode. However, these electrodes require expensive preparation and have lower values of Cs and small working potential limits for the performance of ASC devices. Materials like copper sulfides (CuS, CuS2, etc.) can operate in a wide negative potential window. Therefore, the combination of polymeric gel electrolyte with a copper sulfide electrode will be a good contribution for flexible SC devices. On the contrary, MnO2 has received much attention as a positive electrode to fabricate ASC devices as it shows higher theoretical Cs (1370 F g−1) and has the highest potential window (>+0.8 V). Previously, Gao et al.12 fabricated ASC devices using MnO2/Ni as a positive electrode, Cao et al.13 assembled ASC devices using MnO2 as a positive and graphene as a negative electrode, Wang et al.14 designed ASCs using MnO2 nanotubes and activated carbon nanotubes (CNTs), Attias et al.15 selected MnO2 as a positive electrode for ASCs, Fan et al.16 designed a graphene/MnO2// ACN ASC device, Zhang et al.17 synthesized a MnO2 composite positive electrode for ASCs, and Lei et al.18 grew MnO2 nanofibers on carbon spheres and used them as a positive electrode in ASCs. In addition, the higher conductivity and nonpoisonous nature of CuS make it possible to prepare the negative electrode of ASC devices. Huang et al.19 synthesized CuS/MWCNTs by the hydrothermal process and used them as Received: March 5, 2018 Accepted: April 24, 2018 Published: April 24, 2018 A

DOI: 10.1021/acsami.8b03690 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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Scheme 1. (a) Schematic Diagram Illustrating the 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

diffraction peaks resemble (110), (200), (310), (400), (211), (301), (411), (600), (521), (002), and (741) planes, ratifying the tetragonal phase of α-MnO2 (JCPDS-44 0141). The lowintensity diffraction peaks signify the nanocrystalline nature suitable for SC application.26 Moreover, the XRD pattern of CuS thin films shows crystalline planes such as (101), (102), (103), (105), (106), (110), (108), (202), and (116) of covellite-type hCuS (JCPDS-00-001-1281, hexagonal structure, a = b = 3.791 Å and c = 16.4300 Å). The broader and intense peaks in the XRD pattern authenticate the smaller size of crystallites.27 The peaks shown by asterisk (*) are due to the FSS substrate. Figure 1B,a displays the Fourier transform infrared (FTIR) spectrum of αMnO2 thin films. The peak at 3354 cm−1 denotes the −OH stretching vibrations. The peak at the position of 615 cm−1 accompanies the Mn−O− stretching modes.28 Figure 1B,b shows the FTIR spectrum of the h-CuS film. The peaks that appeared 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 to the stretching mode of the hydroxyl group. These results for the h-CuS film are well matched with the literature.29−31 Moreover, the highly intense peaks of both electrode materials suggest the availability of more numbers of functional groups, which can be helpful for SCs. Figure 1C,D displays X-ray photoelectron spectroscopy (XPS) survey spectra of α-MnO2 and h-CuS materials on the FSS backbone. The difference of energy between Mn 2p3/2 and Mn 2p1/2 is in good agreement with the literature values,32−35 suggesting that only αMnO2 is developed on the FSS substrate. The binding energies of 530 and 530.9 eV are assigned to Mn−O−Mn and Mn−O−H, respectively (Figure 1F).36 The magnified spectrum of Cu 2p showed in Figure 1G illustrates peaks at 932 and 952.3 eV correlating with Cu 2p 3/2 and Cu 2p 1/2 , respectively. Furthermore, two small satellite peaks observed at ∼934.6 and 954.6 eV correspond to the Cu2+ state.37 The binding energies of 162.2 and 169.2 eV related to S 2p3/2 and S 2p1/2 states, respectively, indicate the development of the pure h-CuS phase (Figure 1H). These results are in good agreement with the literature values for h-CuS material.37−39

an electrode; nanostructured CuS networks composed of connected nanoparticles were prepared by Fu et al.20 and used as a 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. The above-mentioned reports have limitations regarding the ED, PD, cycling stability, and cost of ASC devices. These drawbacks, like lower ED, PD, and cycling stability, can be eliminated by replacement of the carbon-based materials with metal sulfide (CuS) electrodes. This combination of cathode (αMnO2) and anode (h-CuS) materials may enhance the electrochemical performance because of the higher conductivity, mechanical stability, and wider potential of both electrode materials. There are a few metal sulfides, such as tin sulfide (SnS)23,24 and molybdenum sulfide (MoS),25 that have a negative potential window with greater electrochemical performance compared to that of carbon-based materials, but they have a lower potential window than that of the CuS electrode. Comparing all these negative electrodes, CuS shows a wider potential window and higher Cs, ED, and PD as well as electrochemical cycling stability. The present work is related to the synthesis of α-MnO2 and hCuS spherical nanostructured electrodes for ASC devices with high mechanical strength, cost-efficient, and flexible stainless steel (FSS) substrates. Our approach is to fabricate ASC devices for higher energy storage using α-MnO2/FSS and h-CuS/FSS electrodes using a gel electrolyte. After obtaining the preliminary basic characterization of electrodes, the electrochemical properties are tested. Series-linked two ASC devices charged up to 30 s and seven white light-emitting diodes (LEDs) are used for the performance demonstration of ASC devices.

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RESULTS AND DISCUSSION Thin-film deposition steps and ASC device fabrication strategies are schematically represented in Scheme 1a,b. It shows the chemical deposition of α-MnO2 and h-CuS electrodes. The X-ray diffraction (XRD) analyses confirm the crystal structure and phase of the deposited materials. Figure 1A displays the XRD of α-MnO2 and h-CuS thin films on FSS substrates. The detected B


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Figure 1. (A) XRD patterns and (B) FTIR 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 h-CuS thin film, (E) Mn 2p spectrum, (F) O 1s spectrum, (G) Cu 2p spectrum, and (H) S 2p spectrum.

Figure 2A,B depicts the field emission scanning electron microscopy (FESEM) images of α-MnO2 thin films at 10k× and 50k×. The only nanosphere is focused in Figure 2B. The diameter of this sphere is about 750 nm. On the outer side of the nanosphere, many interconnected nanorods (average size of 65 nm) are observed. The inset shows a water contact angle of 29° for the α-MnO2 film surface. The transmission electron micrograph of the nanosphere is shown in Figure 2C. One

nanosphere with outer interconnected nanorods is clearly observed in the transmission electron micrograph (TEM). Figure 2D shows a typical high-resolution transmission electron microscopy (HRTEM) image with distinct fringes and interplanar spacings of 0.506 and 0.511 nm consistent with (200) and (110) planes, respectively, which agree well with the earlier outcomes.40 The inset image shows the selected area (electron) diffraction (SAED) pattern of the α-MnO2 film. The C


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Figure 2. (A, B) FESEM images of α-MnO2 thin film at magnifications of 10k× and 50k× (inset shows water contact angle), (C) TEM image of α-MnO2 powder sample, (D) HRTEM image of α-MnO2 sample (inset shows the SAED image of α-MnO2), (E) FESEM image of α-MnO2 thin film at a magnification of 25k×, (F) elemental percentage of Mn and (G) elemental percentage of the O in α-MnO2 thin film, and (H) EDAX spectrum of the αMnO2 thin film (inset table shows the atomic and weight percentages of Mn and O elements).

Na2S2O3 does not control the nanocrystal sizes and only nanospherical particles are developed on the surface of the FSS substrate. The inset image shows the water contact angle of the hCuS thin film, indicating its hydrophilic nature, as it gives 37° angle (inset of Figure 3A). The inset of Figure 3B shows the TEM image of the film surface. The well-defined spherical and elliptical shaped nanoparticles of h-CuS materials are seen in the TEM analysis (Figure 3C). The HRTEM image of the h-CuS thin film is depicted in Figure 3D. The well-defined fringe observed with an interplaner distance of 0.33 nm corresponds to the (102) plane. Figure 3E shows the SAED pattern of the h-CuS thin film. The point positions clearly indicate the hexagonal phase with (100) and (110) planes. Figure 3F,G demonstrate elemental mapping images of Cu and S elements, respectively. It reveals the uniform distribution of Cu and S elements on the film surface. Figure 3H depicts the energy-dispersive X-ray spectrum for the h-CuS electrode, where Cu and S peaks are originated.

well-observed spots denote (200) and (110) planes. This types of morphology are definitely beneficial for the energy storage electrode due to its nanostructure and higher surface area.40−42 Figure 2E shows the FESEM image at 25k× magnification. In accordance with this image, elemental mappings of Mn and O elements are shown in Figure 2F,G, respectively. The energy dispersive X-ray analysis (EDAX) spectrum is shown in Figure 2H. The inset table shows the weight and atomic percentage of elements. This analysis confirms the formation of MnO2 thin film on FSS. Figure 3A,B displays FESEM images of h-CuS thin film at 10k× and 25k×. The shape of the h-CuS nanoparticle is not exactly spherical, but it is a contribution of elliptical as well as spherical nanoparticles. The used precursor and deposition conditions play important roles in surface modifications. The starting materials of CuSO4 (Cu source) and Na2S2O3 (S source) contribute to form nanoparticles. The quick release of S2− from D


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

Figure 4. (A, 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 the α-MnO2 electrode, (C) capacity retention with cycle number plot (inset shows CV curves at a scan rate of 100 mV s−1 for different cycles), and (D) Nyquist plot of the α-MnO2 electrode (inset shows the equivalent circuit from which the Nyquist plot is generated).

thin-film materials are desirable for use in electrode material as charge-storage electrodes in supercapacitor devices. The type IV adsorption−desorption analysis (Figure S1A,B) and the corresponding pore size spreading plots (Figure S1C,D) of α-MnO2 and h-CuS nanostructures support the presence of

The inset table shows the weight (Cu 40.11%, S 59.89%) and atomic (Cu 45.06%, S 54.96%) percentages of Cu and S elements. It discloses that the electrode consists of an h-CuS thin film on FSS substrates with good stoichiometry. Therefore, in conclusion, the surface morphologies of α-MnO2 and h-CuS E


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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 the h-CuS electrode (inset shows equivalent circuit).

Cs values of 505.5, 464, and 333 F g−1 are determined for 4, 5, and 6 mA cm−2, respectively. The nanostructure of the electrode helps in 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, respectively, and are shown in Figure S2C. The electrochemical stability of the 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 the charging− discharging process (Figure 4C). The CV cycles of the electrode are depicted in inset of Figure 4C. For 2000 CV cycles, the αMnO2 electrode exhibits 92.5% capacity retention (7.5% loss of capacitance). Figure 4D shows the Nyquist plot of the electrode in 1 M Na2SO4 aqueous electrolyte (inset displays a fitted equivalent circuit). The charge transfer (Rct) and equivalent series resistances (ESRs) of 2.44 and 2.91 Ω cm−2, respectively, are observed. The overall observed electrochemical parameters (Cs, ED, PD, and cycling stability) of the α-MnO2 electrode demonstrate good electrode potential for ASC devices.48 Electrochemical Supercapacitive Properties of h-CuS/ FSS Electrodes. The CV curves are recorded from a potential of −1.0 to +0.2 V/SCE (Figure 5A). The intercalation/ deintercalation reaction of the electrode electrolyte is as follows49,50

pores on the film surface. The Brunauer−Emmett−Teller (BET) surface areas of α-MnO2 and h-CuS nanostructures are around 75 and 83 m2 g−1, respectively. For the α-MnO2 electrode, pores are distributed on the surface, with the pore radius fluctuating from 5.80 to 25.3 nm. The ranges of pore radii of h-CuS and αMnO2 materials confirm the meso-/macroporous range. Figure S1E,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 and h-CuS electrode materials 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.

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ELECTROCHEMICAL PROPERTIES Electrochemical Supercapacitive Properties of αMnO2/FSS Electrodes. To fabricate an efficient ASC supercapacitor device, choice of the electrode is a very important factor.43−45 Figure 4A shows cyclic voltammetry (CV) curves of the α-MnO2 electrode from 5 to 100 mV s−1. Figure 4B displays gavanostatic charge−discharge (GCD) curves of the α-MnO2 electrode for 4, 5, and 6 mA cm−2. The reversible redox reaction is as follows46 MnO2 + x H+ + y Na + + (x + y)e− ↔ MnOOHxNa y (1)

CuS + OH− ↔ CuSOH + e−

Further, Cs of electrodes is calculated with respect to scan rates as well as current densities (Figure S2A,B). The Cs values of 550, 452, 440, 378, and 316 F g−1 are observed for the α-MnO2 electrode corresponding to 5−100 mV s−1. The approximate values of Cs are detected from the charge−discharge analysis. The

(2)

The charge−discharge curves for the h-CuS electrode were attained in a potential range between −1.0 and +0.2 V/SCE at 1, 4, 5, and 6 mA cm−2 and are represented in Figure S3A. In F


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ACS Applied Materials & Interfaces charging profiles, the position “O” indicates oxidation and position “R” shows reduction correspondence as observed in the CV curves of the electrode. The nonlinear region in the charging curve provides evidence that the h-CuS electrode shows redox reactions. Figure 5B shows the plot of Cs versus the scan rate. The Cs of 885 F g−1 is calculated at 5 mV s−1 for the h-CuS electrode. The Cs of 690 F g−1 is observed for the h-CuS electrode at a current density of 4 mA cm−2 (Figure S3B). Furthermore, the values of ED and PD are calculated from formulae reported in the literature.51,52 The ED and PD of 62 Wh kg−1 and 1.7 kW kg−1, respectively, are reported for the h-CuS electrode and included in the Ragone plot (Figure S3C). The capacity retention versus CV cycle plot is depicted in Figure 5C. The CV cycling stability of the h-CuS electrode is calculated by repeating the CV curves at 100 mV s−1 for 2000 times (inset of Figure 5C). The Cs of the h-CuS electrode is 885 F g−1 for the second cycle; as the cycle number increases, Cs of 812 F g−1 is observed for the 2000th cycle, denoting cycling stability of 92% (8% capacitance loss). Moreover, the conductibility of the prepared h-CuS electrode is premeditated by electrochemical impedance spectroscopy (EIS) measurement and included in the Nyquist plot (Figure 5D). The EIS measurements are carried out at a potential of 10 mV and frequency ranging from 0.1 Hz to 1 MHz. The equivalent circuit of the Nyquist plot is shown in the inset. The plot shows Rs, which is developed due to the internal resistance of the interface, ionic resistance of the electrolyte, and internal resistance of the electrode.53 The Rct of the electrode material is determined by taking the diameter of the semicircle at high frequency on the Nyquist plot. The straight plot after the semicircle indicates Warburg impendence (W) connected to the ion diffusion resistance, whereas intercept on the x-axis stretches Rs.54 For the h-CuS electrode, Rs of 0.9 Ω cm−2 and Rct of 3.96 are detected in 2 M KOH electrolyte, which represent improved electronic conductivity and electroactivity of the h-CuS electrode. The achieved higher performance of the h-CuS electrode can be attributed to the decent conductivity of the FSS substrate (decreasing interfacial resistance) and the nanostructure of the surface of the electrode (enhancing contribution of electroactive material in electrochemical reactions). The nanoparticles of h-CuS surface access the electrolyte ions and provide a small way for intercalation reaction by dropping resistances. Complete electrochemical performance shows the ability of the h-CuS electrode as a negative electrode in the fabrication of ASC supercapacitors. 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 the following equation I = aV

b

Figure 6. (A) Plots of log(anodic current density) against log(scan rate) for α-MnO2 and h-CuS to calculate the b values in the CV curves from 5 to 100 mV s−1. (B, C) Plots of oxidation and reduction peak current versus the square root of the scan rate for α-MnO2 and h-CuS electrodes.

(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, which is either capacitive or diffusion controlled. However, if b = 1, then it suggests that the contribution comes from capacitive behavior, and if b = 0.5, it 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 6A). Figure 6B,C specifies the highest current with the square root V for α-MnO2 and h-CuS electrodes. Ipo and Ipr denote the anodic and cathodic current densities, respectively. 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 αMnO2/FSS/PVA−Na2SO4/α-MnO2/FSS Symmetric Devices. The symmetric α-MnO2/FSS/PVA−Na2SO4/α-MnO2/ FSS device is made up of PVA−Na2SO4 gel electrolyte and αMnO2/FSS electrodes. The appropriate operating potential window of the symmetric device is selected by taking CV curves at different potentials (+0.8 to +1.4 V) (Figure S4A). This potential is also confirmed by GCD analysis. The GCD curves are taken at different potentials of +0.6 to +1.2 V at a constant current of 3 mA (Figure S4B). The maximum potential of +1.2 V G


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ACS Applied Materials & Interfaces Scheme 2. Schematic Representation of Fabrication Stepsa

(A) α-MnO2/FSS/PVA−Na2SO4/α-MnO2/FSS, (B) h-CuS/FSS/PVA−KOH/h-CuS/FSS, and (C, D) α-MnO2/FSS/PVA−LiClO4/h-CuS/FSS ASC devices fabricated by α-MnO2 and h-CuS electrodes using PVA−LiClO4 gel electrolyte. CV curves at 100 mV s−1 for (E) α-MnO2/FSS/PVA− Na2SO4/α-MnO2/FSS (inset shows the photograph of demonstration by glowing red LED), (F) h-CuS/FSS/PVA−KOH/h-CuS/FSS (inset shows photograph of demonstration by glowing red LED), and (G) α-MnO2/FSS/PVA−LiClO4/h-CuS/FSS ASCs (inset shows photograph of demonstration by glowing white LEDs). (H) Comparative CV curves in one plot. a

is achieved by the symmetric supercapacitor device. The charge− discharge curves at 3−5 mA are depicted in Figure S4C. The CV of the symmetric device at 5−100 mV s−1 shows a higher area under CV curve for 100 mV s−1 (Figure S4D). As more electroactive electrode materials contribute to electrochemical reaction at lower scan rate/current density due to availability of time, it gives higher Cs compared with a higher scan rate/current density (Figure S4E,F). The Cs values of 67.3 and 54.6 F g−1 are calculated for the α-MnO2/FSS/PVA−Na2SO4/α-MnO2/FSS symmetric device. The flexibility of the device is tested by measuring CV at different bending positions of the device (Figure S5A). At 180° bending position, the device loses its Cs up to 63 F g−1 (Figure S5B). The Nyquist plot analysis is carried out at 10 mV. The Rs of 0.6 Ω cm−2 and Rct of 7.4 Ω cm−2 are observed for the symmetric device (Figure S5C). The capacity retention with respect to cycle number plot is shown in Figure S5D. Furthermore, CV cycles are taken up to 1500 cycles to test the stability of the symmetric device (inset of Figure S5D). The maximum capacity loss of 12.8% (87.2% stability) is observed after 1500 CV cycles for the FSS symmetric device. The Bode plot analysis and real and imaginary capacitance disparities with applied frequency are depicted in Figure S5E,F. Inset photographs indicate the demonstration of the symmetric device by lighting one LED for the period of 3 min after 30 s of charging. ELectrochemical Supercapacitive Properties of h-CuS/ FSS/PVA−KOH/h-CuS/FSS Symmetric Devices. The electrochemical properties of the 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 selected by considering CV curves from +0.6 to +1.4 V (Figure S6A). Furthermore, GCD curves of the symmetric device are carried

out at a current of 3−5 mA (Figure S6B). The CV curves at 5− 100 mV s−1 are taken at the selected +1.2 V potential (Figure S6C). The Cs of 60.1 F g−1 is observed at 5 mV s−1 (Figure S6D), and 43 F g−1 is calculated at 3 mA (Figure S6E). Inset photograph shows the demonstration of the symmetric device by glowing two yellow and two red LEDs for up to 2 min. Figure S6F shows capacity retention with the cycle plot, and the inset depicts CV curves at different cycle numbers for 2000 cycles. The device shows 84.3% capacity retention for 2000 cycles. Figure S7A shows the Bode plot of the symmetric device. The phase angle of 24.90° is observed for the h-CuS/FSS/PVA−KOH/hCuS/FSS symmetric device. The dissimilarity of capacitance with frequency plots is analyzed using impedance measurements (Figure S7B). The Rs and Rct of 1.93 and 3.17 Ω cm−2, respectively, are observed for the symmetric device (Figure S7C). Flexibility of the device is tested by taking the CV cycles at bending positions of 0, 90, and 180° (Figure S7D). The performances of α-MnO2/FSS/PVA−Na2SO4/α-MnO2/ FSS and h-CuS/FSS/PVA−KOH/h-CuS/FSS symmetric devices are not sufficient to use these devices at the commercial level. Thus, the strategy is to enhance the performance of solid-state devices by designing an asymmetric supercapacitor. Asymmetric design shows advantages over the symmetric one as it provides a wide potential window, cycling stability, and ED. The fabrication steps of α-MnO2/FSS/PVA−Na2SO4/α-MnO2/FSS and hCuS/FSS/PVA−KOH/h-CuS/FSS symmetric devices as well as α-MnO2/FSS/PVA−LiClO4/h-CuS/FSS ASC asymmetric devices are mentioned in Scheme 2A−D. The CV cycles at 100 mV s−1 for the above-mentioned devices are included in Scheme 2E−G. The size difference (area under CV curves) is analyzed by plotting all CV curves in one plot (Scheme 2H). The symmetric devices achieve operating potential up to +1.2 V, and for the H


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

ASC device. This analysis reveals that the highest performance of the ASC device is achieved by using a combination of α-MnO2 and h-CuS electrodes. The GCD curves of the ASC device at currents of 4, 6, 8, and 10 mA are shown in Figure 7F. The highest Cs of 85.6 F g−1 is attained at a current of 4 mA (Figure S8C). The bending positions of the device at 0, 45, 90, 135, and 180° of the ASCs are shown in Figure 7G−K. The obtained capacitance at different bending positions is plotted in Figure S7D. Negligible change in capacitance (overlapping of CV cycles) is observed for the ASC device at different bending positions of the device (inset of Figure S8D). The overlapping of CV curves on each other indicates a negligible effect of bending of device on area under CV curve. The Cs at a bending of 180° is retained up to 98% (only 2% loss). The GCD cycles for h-CuS/FSS/PVA−KOH/h-CuS/FSS, αMnO2/FSS/PVA−LiClO4/α-MnO2/FSS symmetric, and αMnO2/FSS/PVA−LiClO4/h-CuS/FSS ASC devices are plotted in Figure 8A. Figure 8B illustrates the Nyquist plot of the ASC device. The Rct of 2.2 Ω and Rs of 1.2 Ω are observed for ASCs. The electrochemical cycling stability of the ASC supercapacitor device was examined by considering CV curves up to 5000 cycles (Figure 8C). To visualize the cycling stability difference of symmetric and asymmetric SCs, the comparative capacity retention is plotted in Figure 8D. The highest stability of 93.3% is observed for the ASC device compared to that for αMnO2/FSS/PVA−Na2SO4/α-MnO2/FSS (87.2%, 1500 CV cycles) and h-CuS/FSS/PVA−KOH/h-CuS/FSS (84.3%, 2000 CV cycles). The achieved better electrochemical stability can be

asymmetric device, it goes up to +1.8 V potential for a single device. ELectrochemical Supercapacitive Properties of αMnO2/FSS/PVA−LiClO4/h-CuS/FSS ASC Devices. The ASC device is fabricated using α-MnO2/FSS and h-CuS/FSS electrodes, and the individual CV plots of these electrodes are depicted in Figure 7A. Before fabrication of the ASC device, the mass ratio of electrodes was adjusted as 0.46 according to eq 5. Figure 7B depicts CV curves of ASC devices 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 increase up to +1.8 V, indicating that +1.8 V potential is a suitable potential for ASC devices. The nature of CV curves denotes the pseudocapacitive faradaic reactions of α-MnO2 and h-CuS.55,56 The CV curves of ASC devices at 5−100 mV s−1 are displayed in Figure 7C. The CV curves of α-MnO2 and h-CuS electrodes and ASC devices are plotted in Figure S8A. Compared to these two electrodes in aqueous electrolyte, the performance of ASC devices using polymer gel electrolyte is high. The Cs of 109.1 F g−1 is observed at 5 mV s−1 (Figure S8B). The available time at 5 mV s−1 is more than 100 mV s−1; hence, more number of electroactive materials exists at a lower scan rate, resulting in higher Cs. The GCD curves of α-MnO2/FSS, h-CuS/FSS electrodes, and α-MnO2/FSS/PVA−LiClO4/h-CuS/FSS ASC supercapacitor devices are plotted in one graph (Figure 7D). The Columbic efficiency of the ASC device at +1.8 V potential is high (Figure 7E). It is observed that the potential of +1.8 V is suitable for the I


Research Article


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

Figure 9. (A, B) Demonstration photographs during the initial stage and (C) after 7 min for the α-MnO2/FSS/PVA−LiClO4/h-CuS/FSS ASC device using seven white LEDs of the table lamp and (D) Ragone plot of the ASC device (inset shows performance comparison with the literature FESEM images of (a) α-MnO2 and (b) h-CuS and (c) demonstration photograph with a table lamp).

Table 1. Calculated Electrochemical Parameters of α-MnO2, h-CuS, Symmetric, and Asymmetric Supercapacitor Devices Sr. No.

sample

1 2 3 4 5

α-MnO2 h-CuS α-MnO2/FSS/PVA−Na2SO4/α-MnO2/FSS h-CuS/FSS/PVA−KOH/h-CuS/FSS α-MnO2/FSS/PVA−LiClO4/h-CuS/FSS

electrolyte

specific capacitance (F g−1)

energy density (W kg−1)

power density (kW kg−1)

capacity retention (%)

Rct (Ω cm−2)

ESR (Ω cm−2)

1 M Na2SO4 2 M KOH PVA−Na2SO4 PVA−KOH PVA−LiClO4

550 885 67.3 60.1 109.1

61 62 15.3 12.5 18.9

2.1 1.7 13 16 32

92.5 92 87.2 84.3 93.3

2.91 3.96 7.4 3.17 2.2

2.44 0.9 0.6 1.93 1.2

The demonstration of the fabricated ASC device is carried out with a table lamp and two ASC devices. After 30 s charge, during discharging of the ASC device, seven white LEDs glow for more than 7 min. Figure 9A,B displays the photographs captured at discharging of the ASC device. Figure 9C exhibits the photograph of the table lamp after 7 min. The ED and PD

attributed to electrode design, performance of electrode materials, and polymer gel electrolyte used. Inset photographs show the demonstration of symmetric and asymmetric SC devices by glowing different types of LEDs. The Bode plot of ASCs is depicted in Figure 8E. The shape and phase angle confirm the capacitive behavior of the ASC device. J


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Table 2. Comparative Chart for Electrochemical Performance of ASC Devices Reported in the Literature with the Present Study Sr. No

supercapacitor device

electrolyte

specific capacitance (Cs)

power density (kW kg−1)

energy density (Wh kg−1)

H2SO4/PVA gel

175 F g−1 at 0.5 A g−1

6.8

19.7

2 3

graphene (ILCMG)//RuO2-ILCMG MnO2//Fe3O4 CoMoO4/MnO2

0.1 M K2SO4 KOH

20 F g−1 152 F g−1

0.8 0.8

7 54

4

FeWO4/MnO2

5 M LiNO3

5 6 7 8 9 10 11

NiO//α-Fe2O3 NiO//NiCo2O4 GH//MnO2-NF MnO2/graphene MnO2 NT//AC-CNTs MnO2//AC CuO@MnO2//MEGO

PVA/KOH 6 M KOH Na2SO4 1 mol L−1 Na2SO4 2.0 M Li2SO4 0.5 M K2SO4 Na2SO4

12

CoSe2//MnO2

LiCl/PVA gel

13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29

CNTs@NCS@MnO2//AC rGO/MnO2/CB//rGO/CB MnO2//AC sidiatom@MnO2//AGO MnO2//PEDOT MnO2//PPy PANI//carbon Maxsorb PEDOT//carbon Maxsorb MnO2//PANI PPy//carbon Maxsorb carbon aerogel//Co3O4 MnO2//Fe3O4 MnO2//carbon MnO2//AC MnO2//AC GNCC//AC ZnO@MnO2//RGO

Na2SO4 Na2SO4 Na2SO4 PVDF−Na2SO4 KNO3 KNO3 KNO3 KNO3 KNO3 KNO3 KOH−PVA gel K2SO4 K2SO4 Na2SO4 K2SO4 KOH LiCl/PVA gel

30 31 32

CNTs/MnO2//CNTs/PANI ZnO@MnO2//AC α-MnO2/FSS/PVA−LiClO4/hCuS/FSS

Na2SO4/PVP gel ZnAc−PVA PVA−LiClO4

1

57.2 89 41.7 147 137 49.2 F g−1 at 0.25 A g−1 1.77 F cm−3 at 1 mA cm−2 312.5 209 65.1 F g−1 at 0.5 A g−1

57.4 F g−1 at 1 A g−1 21.5 31 269 22.9 288 0.52 F cm−3 at 10 mV s−1 33 109 at 5 mV s−1

electrochemical stability (%)

ref 5

0.951 2.5 10 0.1 0.1 0.338 85.6

12.4 25.99 14.9 25.2 24.8 7.8 22.1

0.282 W cm−3

0.58 mWh cm−3

220 21 0.885 2.22 120 62.8 45.6 54.1 42.1 48.3 0.750 10.2 19 0.1 16 5.6 0.133 W cm−3

27.3 24.3 15.84 23.2 13.5 7.37 11.46 3.82 5.86 7.64 17.9 8.1 17.3 17.1 10 19.5 0.23 mWh cm−3

1.20 6.5 32

24.8 17 18.9

5000 GCD cycles 84 after 10 000 at 3 A g−1 reported for 40 000 cycles 85 after 10 000 cycles 80.2 after 2000 cycles 83.4 after 5000 cycles 96 after 500 cycles 92 after 6000 cycles 85 after 7000 cycles 101.5 after 10 000 cycles 94.8 after 2000 cycles 92.7 after 4000 89 after 1000 88 after 1000 84.8 after 2000

85 after 1000 cycles

94 after 2000 102 after 10000 98.5 after 5000 cycles

97 after 5000 cycles 93.3 after 5000 cycles

7 8 9 10 11 12 13 14 15 57 58 59 60 61 62 63 63 63 63 63 63 64 65 65 66 66 67 68 69 70 this work

electrodes. The potential difference of positive and negative electrodes allows a +1.8 V potential window for ASC devices compared with symmetric devices. The assembled α-MnO2/ FSS/PVA−LiClO4/h-CuS/FSS ASC 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 αMnO2 and h-CuS electrodes for the ASC device demonstrates (seven white LEDs for more than 7 min after 30 s charging) a new approach for a fascinating energy storing ability of ASC devices.

values of the ASC device calculated using eqs 6 and 7 are plotted in Figure 9D. The ED and PD of 18.9 Wh kg−1 and 32 kW kg−1, respectively, are deliberated for the ASC device. The Ragone plots obtained from symmetric and ASC 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 the ASC device compared to those in previous works on MnO2-based ASCs are included in Table 2. All electrochemical parameters of ASCs show higher values compared with those of the symmetric one.

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CONCLUSIONS In this work, for the first time, an asymmetric supercapacitor was fabricated using the positive α-MnO2/FSS and negative h-CuS/ FSS electrodes and a polymer gel electrolyte (PVA−LiClO4). The nanosphere-like surface morphology of α-MnO2 and h-CuS thin films achieved from a simple, binderless, and scalable chemical bath deposition method displays excellent electrochemical properties. The three-dimensional porous nanospheres of α-MnO2 and h-CuS serve as outstanding three-dimensional

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, and 16 h. The blackish-brown α-MnO2 thin films were developed on substrates. The thickness of the thin film gradually increases up to 12 h, and after that overgrowth of the material is observed. The thicknesses of the prepared thin films were observed to be 406, 586, and 576 nm. The mass loadings on the substrate were about 0.23, 0.34, and 0.28 mg (1 cm2 area) for 8, 12, and 16 h time of K



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deposition, respectively. Surface morphologies of the thin films deposited at different deposition times are included in Figure S10A− C. Good-adherent thin films with maximum thickness and mass are observed for 12 h time. After 12 h, due to overgrowth of the electrode material, the drop of material from substrate is observed (Figure S10D). Hence, α-MnO2 thin films deposited at 12 h are 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 of double-distilled water, and then, 0.8 mL of triethylamine (TEA) and 0.15 M Na2S2O3 solutions were added in each beaker (pH ∼ 2.5 (± 0.1)). Deposition of films was carried out at 333, 343, and 353 K for 3 h. Surface morphologies of these three different films are shown in Figure S11A− C, respectively. The rate of chemical reaction alters the surface structure and thickness of the thin films. At 343 K, the highest thickness of the film is observed to be 785 nm, as compared to that at 333 K (602 nm) and 353 K (696 nm). The mass loading on FSS was optimized by changing the 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 S11D. Assembly of α-MnO2/FSS/PVA−Na2SO4/α-MnO2/FSS, h-CuS/ FSS/PVA−KOH/h-CuS/FSS, and α-MnO2/FSS/PVA−LiClO4/hCuS/FSS ASC Devices. The symmetric SC device was designed by two α-MnO2/FSS electrodes and PVA−Na2SO4 solid gel electrolyte (4 g of PVA and 0.852 g of Na2SO4). Similarly, the h-CuS/FSS/PVA− KOH/h-CuS/FSS device was fabricated using two h-CuS/FSS electrodes and 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 were placed in hydraulic press for 24 h at 1 ton pressure. The αMnO2/FSS/PVA−LiClO4/h-CuS/FSS ASC 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 was 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 (FTIR) spectroscopy; X-ray photoelectron spectroscopy (XPS) (VG Multilab 2000); Brunauer− Emmett−Teller (BET) and Barrett−Joyner−Halenda (BJH) (Quantachrome v11.02); field emission scanning electron microscopy (FESEM); 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 an automatic battery cycler (WBCS-3000 and ZIVE-MP1). The Cs values of the electrode materials and ASC device were calculated using the following equation

Cs =

1 MS (V1 − V0)

∫V

V1

I(V ) dV

0

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.8b03690. BET study, Cs versus scan rate and current density plots of α-MnO2/FSS and h-CuS/FSS electrodes, CV curves, capacitance plots, electrochemical properties at different bending angles for α-MnO2/FSS/PVA−Na 2SO4/αMnO2/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 ASC device, Ragone plots for symmetric and asymmetric devices, FESEM images and thickness, mass loading versus deposition time plots of αMnO2 thin films, FESEM images and thickness, mass loading versus deposition temperature plots of h-CuS thin films (PDF)

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*E-mail: [email protected]. Tel: +91-231-2601212. ORCID

Chandrakant D. Lokhande: 0000-0001-6920-6005 Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS 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.

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(5)

(6)

3600 × ED dt

(7)

and

PD =

REFERENCES

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(4)

0.5 × Cs × (V12 − V02) 3.6

AUTHOR INFORMATION

Corresponding Author

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 the following equations

ED =

ASSOCIATED CONTENT

S Supporting Information *

Here, “M” is mass of the material, “S” is scan rate, (V1 − V0) is potential, and “I” is current density. Furthermore, the electrochemical properties of the symmetric and ASC devices were measured using the twoelectrode system. The ratio of masses of α-MnO2/FSS and h-CuS/FSS electrode materials was adjusted using the following equation C . i × V. i M. i = s M .ii Cs. ii × V . ii

Research Article

where V1 − V0 is the potential window. L


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