High Performance All-Solid-State Asymmetric Supercapacitor Device

Research Article Cite This: ACS Sustainable Chem. Eng. 2018, 6, 787−802

pubs.acs.org/journal/ascecg

High Performance All-Solid-State Asymmetric Supercapacitor Device Based on 3D Nanospheres of β‑MnO2 and Nanoflowers of O‑SnS Amar M. Patil,† Vaibhav C. Lokhande,‡ Umakant M. Patil,† Pragati A. Shinde,† and Chandrakant D. Lokhande*,† †

Centre for Interdisciplinary Research, D. Y. Patil University, Kasaba Bavada, Kolhapur, Maharashtra 416006 (MS), India Department of Electronics and computer Engineering, Chonnam National University, Gwangju 500-757, South Korea

‡

S Supporting Information *

ABSTRACT: The current problem of the relatively low energy density and cycling stability of supercapacitors can be effectively addressed by designing an all-solid-state high performance asymmetric supercapacitor (ASSCs) using chemically deposited beta-manganese oxide (β-MnO2) as a positive active electrode and orthorhombic-tin sulfide (O-SnS) as a negative electrode material on a stainless steel (SS) substrate with a poly(vinyl alcohol)-lithium perchlorate (PVA-LiClO4) solid gel polymer electrolyte and as a separator. Time-dependent surface morphological modification and its subsequent effect on electrochemical performance of the O-SnS negative electrode has been examined. Electrodes prepared at deposition time periods of 120, 240, and 360 min provide specific surface areas (SSA) of 36.7, 78.3, and 65.6 m2 g−1, respectively. Superior nanostructures of both the β-MnO2 and O-SnS electrodes offer high specific capacitance (Cs) of 994 and 1203 F g−1 at 5 mV s−1 and energy and power densities (ED and PD) of 83.3, 69.2 Wh kg−1 and 10 and 1.8 kWh kg−1, respectively. A fabricated ASSCs device exhibits Cs of 122 F g−1, ED of 29.8 Wh kg−1, and PD of 1.25 kWh kg−1, with capacity retention of 95.3% up to 5000 charge−discharge cycles. Impressively, such two series assembled ASSCs can light up light-emitting diodes (LEDs) after 30 s of charge. KEYWORDS: β-MnO2, O-SnS, Thin films, Nanostructure, Electrochemical stability, Asymmetric supercapacitor

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INTRODUCTION To meet the growing energy demands of society for human activities and for developing electronic technology, it is essential to accumulate energy from ordinary energy resources and store these energies in appropriate energy storage devices. Consequently, several efforts have been given to fabricate high performance, energy-producing storage devices. Among them, supercapacitors (SCs) are promising for energy storage because of their high power density (PD), long cycling life, and easy fabrication as well as safe operation,1,2 even though they generally have lower energy density (ED) (less than 10 Wh kg−1) than ordinary batteries.3 The strategy to reach a high ED and longer cycling life is to use emerging innovative electrode materials, which have high specific capacitances (Cs) at their wide operating potential windows. Transition metal oxides (pseudocapacitive) such as MnO24 have been the most extensively studied as positive electrodes owing to their low cost, ecofriendly nature, high theoretical Cs, and positive wide potential window (more than +0.8 V). Synthesis of the nanostructure of MnO2 thin films on a stainless steel (SS) substrate is easy, uniform, and highly adherent which helps to enhance the cycling performance of the electrode.5 Sn-based © 2017 American Chemical Society

metal chalcogenide materials are used for different applications such as SCs, lithium ion batteries, and solar cells because of their outstanding electrochemical properties. However, these electrode materials have not been widely studied as SCs materials, whereas they are mainly used in lithium ion batteries owing to a large volume extension. On the other side, in order to choose a negative electrode material, the limitations of easy availablity and low cost negative carbon electrodes (carbon aerogel, activated carbon, and graphite electrodes), with high resistivity owing to contact resistance between carbon particles that cause a rise in internal series resistance, leading to a decrease in electrochemical performance of the negative electrode,6 must be taken into account. Stannous sulfide (SnS) is emerging as a fascinating new electrode material for electrochemical capacitors. SnS and SnS2 are progressively significant due to their special semiconducting properties.7 They also have substantially high chemical stability, thus eluding corrosion in SCs devices. Additionally, Sn and S have Received: September 6, 2017 Revised: November 17, 2017 Published: November 22, 2017 787

DOI: 10.1021/acssuschemeng.7b03136 ACS Sustainable Chem. Eng. 2018, 6, 787−802

Research Article

ACS Sustainable Chemistry & Engineering

Scheme 1. (A) Synthesis Process of β-MnO2 and O-SnS Nanostructures from Chemical Bath Deposition Method (deposition steps indicate nucleation, aggregation, coalescence, and growth of thin film). (B) Bond Structure Alteration during Synthesis of Thin Film Electrodes

Figure 1. (A) XRD pattern of β-MnO2 thin film on SS substrate. (B) XPS survey spectrum. (C) Mn 2p spectrum. (D) O 1s spectrum of β-MnO2 thin film. (E) N2 adsorption−desorption isotherms of β-MnO2 thin film. (F) BJH pore size distribution plot of β-MnO2 powder sample.

ethanolamine ligands. Liu et al.13 synthesized single crystalline SnS nanowires using a cationic surfactant cetyltrimethylammonium bromide (CTAB). Koktysh et al.14 prepared SnS nanocrystals (NCs) from bis(diethyldithiocarbamato) tin(II) in oleylamine at a higher temperature. The ED, indicated as ED = CV2/2, can be enhanced by growing the Cs of active electrode materials or widening the operating potential window (V). Now, two approaches are used to widen the potential window of SCs by using organic electrolytes and fabricating asymmetric supercapacitors.15 Organic electrolytes can offer an improved electrochemical stability but suffer from lower ionic conductivity and toxicity.16 Therefore, an asymmetric solid state supercapacitor (ASSCs) design in gel electrolytes is an effective approach to enhance the

fairly little toxicity and are not dangerous to the environment. The nontoxicity and abundant availability in nature supports the fabrication of SCs devices that are ecologically safe.8,9 Properties like a high adsorption coefficient, wide operating potential window (> −0.7 V/SCE) and high storage capacity with a high SSA fix the scope of the SnS electrode in future SCs. The electrochemical supercapacitive performance of the SnS electrode material can be further improved by increasing the electric conductivity, porous surface, and potential windows of the electrode.9 Liu et al.10 synthesized single-crystalline SnS nanobelts via a simple wet-chemical route. Li et al.11 prepared SnS nanoparticles mechnochemically and then coheated them with poly(vinyl alcohol) at different temperatures to get a carbon coating. Xu et al.12 prepared SnS nanocrystals using 788


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Figure 2. (A, B, and C) FE-SEM images of β-MnO2 thin film surface at magnifications of 10,000 X, 20,000 X, and 50,000 X, respectively. (D and E) elemental mapping analysis showing Mn element percentage and O percentage, respectively. (F) EDS spectrum of β-MnO2 film surface.

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RESULTS AND DISCUSSION The synthesis processes of positive β-MnO2 and negative OSnS electrodes are shown in Scheme 1. The preparation of thin films with steps like nucleation, aggregation, coalescence, and growth is displayed schematically. The reaction mechanisms of the synthesis of films are explained with drawing bond diagrams. Figure 1(A) displays the XRD pattern of a chemical bath-deposited (CBD) MnO2 thin film. The observed diffraction peaks correspond to (110), (211), and (002) planes, according to the standard characteristic diffractions of β-type tetragonal MnO2 (JCPDS card no. 24-0735).26 No other diffraction peaks are observed than SS substrate peaks, which resemble the pure phase formation of β-MnO2. The wide and low intense diffraction peaks denote the nanocrystalline nature of β-MnO2, which is beneficial for SCs application.27 This structure offers rapid ion diffusion in electro-active materials, which increases the charge storing capacity of the SCs. The peaks marked by an asterisk (∗) correspond to the SS substrate. The FTIR spectrum shows the intense peak at 3354 cm−1, which is related to −OH stretching vibrations of MnO2 (Figure S1). The absorption peaks at 1640 and 1405 cm−1 are accredited to −C−O bending vibrations joining with Mn atoms. The absorption peak at 615 cm−1 is accompanied with the coupling mode of the Mn−O stretching modes.28 Additionally, complete information about chemical compositions and electronic states of β-MnO2 is investigated through an XPS study. Figure 1(B) shows the survey spectrum of the βMnO2 thin film material. From vigilant evaluation of the XPS survey spectrum, it is demonstrated that the peaks of Mn, O, and C elements are present in the sample material. The detected feeble peak of C is due to the carbon dioxide molecules present in air and hydrocarbons from the XPS machine.29 The Mn 2p peak can be divided into the 2p3/2 and 2p1/2 peaks at binding energies of 642.3 and 653.9 eV (Figure 1(C)), respectively, and denotes that the chemical state of element Mn is in the Mn4+ state.30 The separation of energy (11.8 eV) between the two states of Mn 2p3/2 and Mn 2p1/2 is well in accordance with the literature,31 suggesting that β-

operating potential window and afford a maximum ED. In the efforts to enhance ED, MnO2-based ASSCs such as MWCNT/ MnO 2 //Fe 2 O 3 , 17 Ni/GF/MnO 2 //Ni/GF/Ppy, 18 Al@Ni@ MnOx//GF,19 MnO2@PANI//GF,20 α-MnO2@NiCo2O4,21 CoMoO4/MnO2,22 FeWO4/MnO2,23 and Ni−Co−B//CAC24 and SnS-based ASSCs such as SnS2/RGO25 have been reported previously. On the other hand, the carbon electrode shows lower Cs and high cost, which discards the possibility of using a carbon material as a negative electrode compared with an SnS electrode. Therefore, it is reasonable to assume that β-MnO2/ SS//O-SnS/SS ASSCs would possess more excellent electrochemical performance than that of β-MnO2- and O-SnS-based other ASSCs devices. Until now, no reports are available for fabrication of high performance the SS/β-MnO2//O-SnS/SS ASSCs device. The present work succeeds in coalescing all the discussed advantages of the preparation of MnO2-nanosphere- and SnSnanoflower-nanostructured binder-free electrodes for ASSCs directly on cost efficiency, high mechanical strength, excellent flexibility, and stability in the acidic electrolyte SS substrate. Our strategy is to develop an ASSCs device to achieve a wide potential window, longer cycling life, and higher energy storage SCs using the above-discussed positive (β-MnO2, optimizing results in a Cs of 994 Fg1− at 5 mV s−1, ED and PD of 83 Wh kg−1 and 10 kW kg−1 and capacity retention of 94.3% over 3000 cycles) and negative (O-SnS, optimizing results in a Cs of 1203 Fg1− at 5 mV s−1, ED and PD of 69.2 Wh kg−1 and 1.8 kW kg−1 and capacity retention of 90.2% over 2000 cycles) electrodes using a PVA-LiClO4 polymer gel electrolyte. After completion of the initial basic characterizations of electrodes, electrochemical properties are tested using a battery cycler and electrochemical workstation units. Series-connected two ASSCs devices charged for 30 s and four yellow, four red, one yellow, and one green LEDs were used for demonstration of application of ASSCs 789


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Figure 3. (A) XRD patterns of (a) TS1, (b) TS2, and (c) TS3 thin films on SS substrates. (B) FTIR spectra of (a) TS1, (b) TS2. and (c) TS3 thin films. XPS spectra of TS2 thin film of (C) survey scan spectrum, (D) Sn 3d core level spectrum, and (E) S 2p core level spectrum. (F) N2 adsorption−desorption isotherms of (a) TS1, (b) TS2, and (c) TS3 thin films prepared at different deposition time periods. Inset shows pore size distribution plot of TS2 powder sample.

ture.33 The elemental mapping of Mn and O elements are depicted in Figure 2(D and E), respectively. Figure 2(F) shows the energy dispersive X-ray spectrum for the β-MnO2 thin film. In the spectrum, Mn and O element peaks are originated. Some peaks other than Mn and O are observed due to the SS substrate. It reveals that the electrode consists of Mn and O elements, confirming the formation of MnO2 on the SS substrate. In order to prepare the best performance negative electrode, three different nanostructures of SnS thin films are achieved basically by varying the deposition time. Sn ions are released from a 0.1 M SnCl2 solution and form a complex with triethylamine (TEA) (eqs 1 and 2). Na2S2O3 gives H2S in double distilled water (DDW) (eq 3). The complex of Sn2+ ions with TEA and H2S after addition of HCl gives SnS thin films on an SS substrate at 343 K (eq 4) (Scheme 1).

MnO2 is developed on the SS substrate. Figure 1(D) represents an XPS spectrum of O 1s showing that peaks of O 1s at 529.68 and 531.3 eV are allocated to Mn−O−Mn and Mn−O−H, respectively.32 The nitrogen adsorption−desorption isotherms and corresponding pore size distribution plot of the β-MnO2 nanostructure confirms the porous surface (Figure 1(E and F)). These are typical type IV adsorption−desorption isotherms with a hysteresis loop characteristic for the MnO2 electrode material. The BET SSA of the β-MnO2 nanostructures is about 75 m2 g−1, and pores are distributed overall on the surface with the pore radius ranging from 1.83 to 12.39 nm, confirming the meso/macroporous range, which is supportive to storage of charge in porous interfaces. The surface morphology investigations of the β-MnO2 thin film on the SS substrate are carried out through FE-SEM analysis. Figure 2(A, B, and C) shows FE-SEM images of the βMnO2 thin film at magnifications of 10,000 X, 25,000 X, and 50,000 X, respectively. The spherical-shaped β-MnO2 nanoparticles are distributed overall on the substrate surface. The high resolution transmission electron micrographs of these nanospheres are shown in Figure S2. A single nanosphere is shown in Figure 2(C), and the average diameter of this single nanosphere is about 550 nm. The average size of the overall nanospheres grown on the film surface is about 350 nm. The nanospheres are distributed overall on the surface of the SS substrate showing different sizes of nanospheres. The contribution of the electroactive material in the electrochemical reaction is an effective parameter to enhance storage capacity of SCs. This type of porous surface and nanostructure is beneficial for energy storage. The nanospheres are connected to each other, building a uniform porous layer via an interconnected structure, which rapidly transfers charge through nanostruc-

SnCl 2·2H 2O + H 2O → Sn 2 + + Cl −2 + 3H 2O

(1)

Sn 2 + + Cl −2 + TEA → [Sn[TEA]]2 + + Cl −2

(2)

Na 2S2 O3 + H 2O → H 2S + Na 2SO4

(3)

[Sn[TEA]]2 + + H 2S + HCl → SnS ↓ +TEA ↓ +HCl ↓ +2H+↑

(4)

The XRD patterns of SnS (TS1, TS2, and TS3) thin films are shown in Figure 3(A). Among the (011), (012), (102), (110), (013), (104), (022), and (115) crystalline planes, the (013) plane shows a high intense peak, and XRD data matches well with JCPDS card no. 00-001-0984. The detected peak positions and crystal planes belong to the formation of orthorhombic structure of SnS (O-SnS) thin films. The surface structure and 790


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Figure 4. FE-SEM images of (A) TS1, (B) TS2, and (C) TS3 thin films prepared at 120, 240, and 360 K at magnification of 10,000 X (inset image shows the water contact angle of TS1, TS2, and TS3 thin films, respectively). Energy dispersive spectroscopy analysis for detection of (D) Sn and (E) S elements present in TS2 thin film. (F) EDS spectrum of TS2 nanostructure. (G) TEM image of TS2 powder sample scratched from TS2 film (inset shows HRTEM image of TS2 film).

growth directions of O-SnS thin films can be controlled by deposition time period and nucleation rate of film formation. At the reaction time of 240 min, the strong diffraction peaks are observed at (013), (104), and (115) planes. The reduction in intensity of peaks for TS2 and TS3 thin films indicates controlled growth of nanoparticles. The presence of sharp diffraction peaks and lack of impurity peaks designates that the O-SnS thin films have high crystallinity.34 The SS substrate peak is denoted by “Δ”. FT-IR spectra of TS1, TS2, and TS3 thin films are displayed in Figure 3(B), and peaks at wavenumbers of 3750, 1606, 1350, 832, and 640 cm−1 support the formation of SnS material on the SS substrate. The peaks observed at 640 and 832 cm−1 in the spectrum are due to the presence of Sn−S bonds.35 The two smaller peaks at 1350 and 1606 cm−1 are due to the formation of C−O and C−H bonds, respectively. The peak appearing at 3750 cm−1 is associated with a stretching mode, which is ascribed to the hydroxyl group. It is clearly seen that the strength and perceptiveness of peaks drop with a rise in deposition time. The intensity of peaks at the 240 and 360 min deposition time shows a number of functional groups present in adherence of the O-SnS thin films. The wide scan XPS spectrum of the O-SnS thin film prepared at the deposition time of 240 min is shown in Figure 3(C). The spectrum shows peaks that were assigned to the Sn 3d3/2, Sn 3d5/2, S 2p3/2, C 1s, and O 1s states. The occurrence of the O 1s peak in the thin film layer is probably due to surface contamination in the open air. The XPS peaks are standardized against the C 1s peak present at 284.6 eV in the O-SnS film layers. The Sn 3d spectrum shown inFigure 3(D) exhibits two main peaks corresponding to Sn 3d5/2 and Sn 3d3/2 that are positioned at 486.5 and 495 eV, respectively. The energy separation among these two peaks is about ∼8.5 eV, which well

agrees with that reported in the literature.36 The observed binding energies of the two peaks are dissimilar from those of elemental Sn in SnO2, SnS2, and Sn2S3; this designates Sn in the SnS phase and +2 oxidation states. In addition to these main peaks, the S 2p3/2 state is observed at 168.6 eV (Figure 3(E)). This confirms the occurrence of only the O-SnS phase in the film layers. The higher SSA and suitable pore volume size of the synthesized electrode materials are important for higher electrochemical performance, which delivers the larger electroactive sites in electrochemical reactions, and the suitable pore volume offers an easy way for an intercalation/deintercalation reaction. The N2 adsorption−desorption isotherm of TS1, TS2, and TS3 powder samples are portrayed in Figure 3(F). Each isotherm curve of the TS1, TS2, and TS3 samples reveals a typical type III isotherm with an H3 hysteresis loop.37 The observed BET SSA of the TS1, TS2, and TS3 samples are 36.7, 78.3, and 65.6 m2 g−1, respectively. The corresponding BJH plot for TS2 is embodied in the inset, and peak positions at 3.05, 5.5, 12.3, 19.6, 23.4, and 33.5 nm in the pore size distribution graph depict the meso/macroporous range of the pores. Chauhan et al.7 reported synthesis of tin sulfide nanoparticles using the solvothermal route for applications in SCs devices. The nanoflowers of the TS2 sample surface show higher SSA with a smaller pore size of 3.05 nm. The better SSA of the TS2 electrode is helpful for more contributiosn of electro-active electrode materials in electrochemical reactions. The peak positions between 3 to 34 nm in the pore size distribution plot designate that the dissimilar-sized meso/ macroporous materials are available in the electrode material. Figure 4(A−C) depicts the surface morphologies of the TS1, TS2, and TS3 thin films at magnifications of 10,000 X, which 791


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Figure 5. (A) Ragone plot. (B) GCD cycles. (C) Capacity retention with cycle number plot (inset shows electrochemical cycling stability curves at 10 mA cm−2 for first and 3000th GCD cycle. (D) Enlarged Nyquist plot of β-MnO2 electrode (inset shows equivalent circuit from which Nyquist plot is generated).

water contact angles of TS1, TS2, and TS3 thin films are included in the inset of Figure 4(A, B, C), respectively. The contact angles of 53°, 36°, and 64° and surface free energies of 65.35, 86.6, and 55.26 (±0.1) (mJ m−2)−2 are observed for TS1, TS2, and TS3 thin films, respectively. The TS2 electrode stretches its hydrophilic nature as it shows the lower contact angle useful to improve the interaction of the electrode with electrolyte for resultant higher SCs performance of the electrode. Figure 4(D and E) depicts the corresponding elemental analysis of TS2 and illustrates Sn and S elements with an atomic ratio near 1:1. The green and red color images are linked with the Sn and S elements, respectively, which is obtained from L- and K-line spectra of the elements. The EDS spectrum shows the presence of Sn and S elements in a thin film material as it displays high intense peaks of Sn and S near same percentage (Figure 4(F)). There are some very small impurity peaks, which are ascribed to the SS substrate and air contamination of the thin film. The TEM images of the TS2 nanoflowers are described in Figure 4(G), and the HRTEM image of the TS2 nanostructure is shown in the inset. The lattice fringes are visibly observed with a d-spacing of 0.26 nm, corresponding to the (013) plane of O-SnS. The Cs values of β-MnO2, O-SnS, and β-MnO2/SS//O-SnS/ SS ASSCs devices from scan rates is calculated using the following equation:

show that O-SnS nanostructures are homogeneously grown and nanoparticles are interconnected with each other on the entire SS substrate. It is observed that three different morphologies such as nanoflakes, nanoflowers, and nanogranules are formed for TS1, TS2, and TS3, respectively. The TS1 thin film shows the formation of a large number of regularly spread nanoflakes with rough and inferior porous structures (Figure 4(A)). At a deposition time of 240 min, the growth rate alters the surface morphology of thin film from nanoflakes to interconnected nanoflowers (Figure 4(B)). It is clearly observed that the size of the nanoflakes reduces with formation of nanoflowers (each has a size about 563 nm), which increases the porous surface for TS2. Furthermore, as deposition time increases, the growth rate of the reaction increases with the nucleation rate, and the surface converts inactive growth with overgrowth of material on an SS substrate at a deposition time of 360 min (Figure 4(C)). At lower magnification, one can see that the entire SS substrate is covered by O-SnS nanogranules on the substrate. The diameter of the nanogranules is between 750−900 nm. The alteration in deposition rate and direction of film formation indicates that the deposition time is a tool to change the surface morphology. The nanoflower-shaped surface offers more SSA in electrochemical reactions.38 Nanoflowers of the TS2 film maximizes the electro-active materials utilization ratio and drops the diffusion length significantly causing high Cs and excellent electrochemical cycling stability. Hence, TS2 nanoflowers are beneficial for further electrochemical investigations. The interfacial contact properties of electrodes and aqueous electrolyte are estimated by contact angle measurement. The 792


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ACS Sustainable Chemistry & Engineering 1 ms(V1 − V0)

Cs =

∫

V1

50 to 250 GCD cycles, the stability increases up to 110.78%, and afterward, it decreases to 94.3% at 3000 cycles (Figure 5(C)). The overlapping of GCD cycles on each other indicate better cycling stability of the electrode material. The magnified Nyquist plots of the β-MnO2 electrode material in a 1 M Na2SO4 electrolyte solution are shown in Figure 5(D). The inset shows an equivalent circuit diagram fitted with a Nyquist plot. The Nyquist plot analysis gives a charge transfer resistance (Rct) and equivalent series resistance (ESR) of 3.9 and 0.62 Ω cm−2, respectively. Lower values of resistance are beneficial for enhancing the storage capacity of the electrode material.42 The phase angle at a higher frequency range is inferior because the ionic resistance of the electrolyte corresponds to lower capacitance (Figure S3(E)). The phase angle increases toward −90° at a lower frequency range denoting the capacitive behavior of the β-MnO2 electrode. The overall performance of the β-MnO2 electrode shows a promising positive electrode material for the ASSCs supercapacitor device. On the other side, the TS1, TS2, and TS3 electrodes have been employed to CV, GCD, and EIS tests to examine the influence of deposition time on the electrochemical supercapacitor performance of the negative electrode. CV plots of bare SS, TS1, TS2, and TS3 electrodes within a potential range from −0.5 to −1.2 V/SCE in a 2 M KOH electrolyte solution at a scan rate of 100 mV s−1 are shown in Figure S4(A). The current densities of TS1, TS2, and TS3 electrodes are greater than the bare SS substrate, indicating that the contribution of the SS substrate to the Cs of O-SnS is negligible. The area under the CV curve increases with an increase in deposition time up to 240 min. The performance achieved is dependent on the morphologies, signifying that the nanoflower-like morphology offers more SSA fast charge intercalation/deintercalation reactions for the TS2 electrode. The CV curves of the TS2 electrode for the scan rates of 5, 10, 50, and 100 mV s−1 are depicted in Figure S4(B). The CV curves display rectangular shapes with small noticeable redox peaks at the potential positions of −0.6 and −1.0 V/SCE, indicating that the charge storage is due to redox reactions as well as EDLC type reactions. The kinetic irreversibility of the electrode shows asymmetry on the oxidation and reduction sides of the CV curves.43 The rise in area under the CV curve with a scan rate is clearly observed for each electrode. The intercalation of OH− ions of the KOH electrolyte to the surface of O-SnS is carried out at charging, and a similar deintercalation takes place during discharging. The charge transfer in the O-SnS electrode material is based on the following reaction:

I(V )dV (5) (5)

V0 2

where Cs is specific capacitance, m is mass on a 1 cm area, s is scan rate, (V1 − V0) is potential window, and I is current response. The Cs values are determined from GCD analysis using following equation: Cs =

I×t m × ΔV

(6)

The charge storage and charge−discharge capacity of electrode materials are analyzed with ED and PD. These values are calculated using the two relations below: ED =

(V12 − V02) × 0.5 × Cs 3.6

(7)

PD =

3600 × ED dt

(8)

where V1−V0 is potential window, Cs is specific capacitance (F g−1), I is current density, m is mass of material loaded at 1 cm2, t is discharging time, and V is potential window. In order to fabricate an efficient ASSCs device, the electrode selection is a primary stage. The ideal electrochemical properties of the β-MnO2 electrode material proves it is a promising positive electrode in ASSCs devices. The electrochemical SCs properties of the β-MnO2 electrode are examined using the CV, GCD, and EIS measurements in a 1 M Na2SO4 electrolyte. The ideal SCs behaviors such as rectangular shape of the CV curve, symmetry in both anodic and cathode sides of the CV curve, and higher area under the CV curve is accomplished by the prepared β-MnO2 electrode (Figure S3(A)). These properties of an electrode confirm that the storage of electric energy is mostly pseudocapacitive.39 As the scan rate increases, the area under the CV curve also increases, which designates effective consumption of an active β-MnO2 material with electrolyte ions in SCs. GCD analysis of the electrode for different current densities of 2, 3, 4, and 5 mA cm−2 (Figure S3(B)) confirms that the charge storage contribution comes from the surface adsorption/desorption of electrolyte ions, and the intercalation/deintercalation of protons in the electrode material means reversible redox reactions.40 The Cs of 994, 733, 625, 406, and 345 F g−1 corresponding to various scan rates (5−100 mV s−1) and 750, 600, 428, and 375 F g−1 for current densities of 2, 3, 4, and 5 mA cm−2 are observed for the β-MnO2 electrode (area of 1 cm × 1 cm) (Figure S3(C and D)). At a lower scan rate and current density, the rate of the intercalation/deintercalation reaction is longer, which may be attributed to the maximum utilization of the electro-active material in electrochemical reactions providing higher Cs as a nanostructure of the β-MnO2 electrode material facilitates an easy way for intercalation/deintercalation of electrolyte ions by reducing diffusion resistance.41 Figure 5(A) shows the Ragone plot of the β-MnO2 electrode material which displays the highest ED and PD of 83 and 10 kW kg−1, respectively. Cycling life of the electrode was measured by 3000 GCD cycles between potential from 0 and +0.8 V/SCE at a current density of 5 mA cm−2 (Figure 5(B)). As the electrode contributes more material in an electrochemical reaction from

SnS + OH− ↔ SnSOH + e−

(9)

The values of Cs for TS1, TS2, and TS3 electrodes at a scan rate of 5 mV s−1are 950, 1203, and 1055 F g−1, respectively. Figure S4(C) shows the Cs versus scan rate plots for TS1, TS2, and TS3 electrodes. The Cs values are greater at a lower scan rate which is attributable to the extra time available to intercalate OH− ions at a lower scan rate than higher scan rate. On the opposite side, at a higher scan rate, only the surface of the electrode material contributes to the electrochemical reaction, not inside material, which provides a lower Cs. The GCD curves at an identical current density of 1 mA cm−2 for TS1, TS2, and TS3 electrodes are synthesized at three different deposition time periods and are shown in Figure S4(D). A higher discharge time is observed for the TS2 electrode. Furthermore, the GCD analysis is carried out for TS2 at different current densities of 1, 2, and 3 mA cm−2 in potential 793


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Figure 6. (A) Ragone plots. (B) CV curves at different cycles for (a) TS1, (b) TS2, and (c) TS3 electrodes at scan rate of 100 mV s−1. (C) Capacity retention versus cycle number plots of TS1, TS2, and TS3 electrodes. (D) Nyquist plots of TS1, TS2, and TS3 electrodes (inset shows best fitted equivalent circuit).

Table 1. Electrochemical Properties of Positive and Negative Electrodes of ASSCs Sr. No

Sample

Morphology

1 2 3 4

β-MnO2 TS1 TS2 TS3

Nanosphere Nanoflake Nanoflower Nanogranule

Electrolyte 1 2 2 2

M M M M

Na2SO4 KOH KOH KOH

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)

994 950 1203 1055

83.3 58.8 69.2 60.5

10 10.5 8.5 10.1

94.3 86 90.2 88.6

3.9 3.88 1.18 1.85

0.62 0.85 0.15 0.46

range from −1.2 to −0.5 V/SCE (Figure S4(E), indicating that the capacitance is contributed to not only from pseudocapacitance but also from an EDLC type. The minimum interface resistance (IR) drop is detected for the TS2 electrode as compared to TS3 and TS1 electrodes. The nonlinear performance of the charge−discharge curve designates the electrochemical adsorption and desorption reactions at the interface of the electrode and electrolyte. The Cs values attained for the TS1, TS2, and TS3 electrodes are 846, 1101, and 900 F g−1, respectively, at a current density of 1 mAcm−2 (Figure S4(F), confirming that the discharge rate of the TS2 electrode has a higher charge storage ability than other electrodes. Ragone plots of TS1, TS2, and TS3 electrodes are shown in Figure 6(A), exhibiting a maximum ED of 58.8, 69.2, and 60.5 Wh kg−1 and PD of 10.5, 8.5, and 10.1 kW kg−1, respectively, which are relatively remarkable compared to the earlier reports on SnS (1.49 Wh kg1− at 0.24 kW kg1−) and SnS2/GO (16.67 Wh kg1− at 0.48 kW kg1−).25 The electrochemical stability of the TS2 electrodes is tested by the CV cycling of electrode material at a scan rate of 100 mV s−1 (Figure 6(B)). The area under the CV curve for each cycle does not reduce further, which indicates the higher capability of the electrode material up to thousands of CV cycles. Figure 6(C) represents the capacity retention versus cycle number plot of TS1, TS2, and TS3 electrodes, and the observed percent-wise

electrochemical stability of TS1, TS2, and TS3 electrode materials is 86%, 90.2%, and 88.6%, respectively. Figure 6(D) shows the Nyquist plots of TS1, TS2, and TS3 electrodes in the frequency range of 10 kHz to 100 mHz at a bias potential of 5 mV. The inset figure shows the equivalent circuit diagram from which the impedance plot is fitted. The Nyquist plots of the TS1, TS2, and TS3 electrodes show a semicircle at a higher frequency region and intercept on the real axis stretches ESR of an electrode material, which contains electronic and ionic contributions.44,45 The ESRs of TS1, TS2, and TS3 electrodes are calculated as 0.85, 0.15, and 0.46 Ω cm−2, respectively. The lower ESR values are observed because of the highly porous structure, which causes low impedance and provides easy access to electrolyte ions for intercalation and deintercalation. The diameter of the semicircle gives Rct values of 3.88, 1.18, and 1.85 Ω cm−2 for TS1, TS2, and TS3 electrodes, respectively. The diffusion of Sn2+ in an electrolyte gives a straight line after a semicircle at 45°, which describes the Warburg’s constant (W). The TS2 electrode displays lower impedance than the TS1 and TS3 electrodes. Hence, the nanoflowers morphological TS2 electrode is suitable for negative electrodes in ASSCs devices (Table 1). The structural changes of the β-MnO2 and O-SnS electrodes after long-term cycling are studied by XRD analysis. Figure 7(A and B) shows the XRD patterns of the electrodes of β-MnO2 794


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Figure 7. XRD patterns of (A) β-MnO2 cathode and (B) O-SnS anode before and after cycling. Nyquist plots of (C) β-MnO2 electrode and (D) OSnS electrode before and after cycling.

Scheme 2. Schematic Representation of Fabrication Process of ASSCs Device

and O-SnS before and after cycling, respectively. The SS substrate peaks are denoted by an “∗” asterisk. After cycling,

both of the electrodes show all of the XRD peaks corresponding to the β-MnO2 and O-SnS phases with a 795


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Figure 8. (A) CV curves of β-MnO2 and O-SnS electrodes at 100 mV s−1. (B) Potential window selection CV plots at different voltages ranging from +1.2 to +1.8 V of β-MnO2/SS//O-SnS/SS ASSCs device. (C) CV curves at different scan rates (5−100 mV s−1) of β-MnO2/SS//O-SnS/SS ASSCs device. (D) Cs versus scan rate plot of β-MnO2/SS//O-SnS/SS ASSCs. Inset shows Cs versus current density plot of ASSCs device.

The plots of Cs versus scan rate and current for ASSCs devices are included in Figure 8(D), which show the maximum Cs of 122 F g−1 at 5 mV s−1 scan rate. The comparative GCD curves of β-MnO2 and O-SnS electrodes and β-MnO2/SS//O-SnS/SS ASSCs device ares depicted in Figure 9(A). The GCD curves of the β-MnO2/SS// O-SnS/SS ASSCs device at different potentials ranging from +0.8 to +1.6 V at a current response of 8 mA are shown in Figure 9(B). The columbic efficiency of the devices at a +1.6 V potential is high which confirms that this potential is suitable for the β-MnO2/SS//O-SnS/SS ASSCs device. The GCD curves of the β-MnO2/SS//O-SnS/SS ASSCs device at currents of 8, 10, and 12 mA are shown in Figure 9(C). The CV curves are taken at different bent positions of the device at bending angles of 0°, 45°, 90°, and 180°. The overlapping of CV curves on each other at all bending angles indicate a negligible effect (2% Cs loss) of bending on areas under the CV curve (Figure 9(D)). The electrochemical cycling stability of the β-MnO2/SS//OSnS/SS ASSCs device is examined by taking GCD curves up to 5000 cycles. Figure 10(A and B) shows the first and 5000th GCD cycles. The shape of the GCD curve remains equivalent for the first and 5000th GCD cycles indicating more capacity retention. The capacity retention with the GCD cycle number plot shown in Figure 10(C) depicts that the capacity retention of 95.3% remains up to 5000 GCD cycles. The inset shows nearly overlapped first and 5000th GCD cycles taken at 10 mA. The achieved better electrochemical stability performance is due to the capacity of electrode materials and used polymer gel electrolyte. Likewise, the EIS analysis of the β-MnO2/SS//OSnS/SS ASSCs device performed at open circuit potential (Figure 10(D and E)). The semicircle is observed at a high

slightly decreased peak intensity. The slight decrease in peak intensity is attributed to the electrochemical dissolution of βMnO2 and O-SnS in a Na2SO4 electrolyte. The present results clearly highlighted the significant structural stability of both electrodes. Figure 7(C and D) shows Nyquist plots of β-MnO2 and OSnS electrodes before and after electrochemical cycling. Both of the β-MnO2 and O-SnS electrodes demonstrated similar Nyquist plots in the higher frequencies. The careful observation of semicircles in the higher frequencies revealed the charge transfer resistance (Rct) of β-MnO2 and O-SnS electrodes changes from 3.9 to 4.8 and 1.18 to 2.15 Ω.cm−2, respectively. Both of the electrodes demonstrated stable solution resistance (Rs) before and after cycling. The negligible change in Rct values and stable Rs clearly highlighted that both β-MnO2 and O-SnS electrodes are stable after electrochemical cycling. The minute change in Rct may arise due to the slight decrease in crystallinity of electrode material. After optimization of high performance electrodes, the βMnO2/SS//O-SnS/SS ASSC devices were fabricated using positive β-MnO2/SS and negative O-SnS/SS electrodes (Scheme 2). CV curves of β-MnO2/SS and O-SnS/SS electrodes for different voltage ranges at a scan rate of 100 mV s−1 denote that a combination of these two electrodes can achieve a large potential as a asymmetric device (Figure. 8(A)). The CV curves of the β-MnO2/SS //O-SnS/SS ASSCs device at different potentials ranging from +1.2 to +1.8 V are shown in Figure 8(B). At + 1.6 V, the device shows the higher current response as well as the area under the CV curve with respect to potential; so, a +1.6 V potential is selected for further electrochemical investigations. CV curves at different scan rates ranging from 5 to 100 mV s−1 are shown in Figure 8(C). 796


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Figure 9. (A) GCD curves of β-MnO2 and O-SnS electrodes and β-MnO2/SS//O-SnS/SS ASSCs device at same current of 8 mA. (B) GCD curves of ASSCs devices at 8 mA for different potentials (+0.8 to +1.6 V). (C) GCD curves of β-MnO2/SS//O-SnS/SS ASSCs device at different currents (8, 10, and 12 mA). (D) CV curves of ASSCs devices at different bending angles at 100 mV s−1.

and other aqueous, solid state supercapacitors.57 The maximum ED and PD of 29.8 Wh kg−1 and 1.25 kW kg−1 are observed for ASSCs device. The demonstration of the fabricated β-MnO2/SS//O-SnS/ SS ASSCs device is carried out with yellow, red, and green light-emitting diodes (LEDs) with a series connection of two ASSCs devices. One ASSCs device has +1.6 V operating potential windows; so, the series combination of the ASSCs device gives a total potential up to +3.2 V. Initially, these ASSCs devices are charged with a potential of +3.2 V up to 30 s, and the ASSCs device is discharged through four yellow, four red, one yellow, and one green LEDs which glow with high intensity up to 2, 3, and 4 min, respectively. The inset of Figure 11 shows initial photographs captured during discharging of the ASSCs device after 30 s charging. The discharging period of the ASSCs device indicates excellent storing ability of the β-MnO2/ SS//O-SnS/SS ASSCs device with better ED as well as PD.

frequency range, and after that, the straight line at the lower frequency range indicates Warburg’s resistance (W). The diameter of the semicircle gives an Rct of 0.24 Ω, and the first intercept on the real x-axis gives an Rs of 0.22 Ω. The lower values of resistance are beneficial for electrochemical performance of the β-MnO2/SS//O-SnS/SS ASSCs device. The phase angle at a higher frequency range is lower because the internal resistance of the electrolyte corresponds to lower capacitance, and the phase angle shifts toward −90° at the lower frequency range specifies the capacitive behavior of the β-MnO2/SS//OSnS/SS ASSCs device (Figure 10(F)). The ED and PD values of β-MnO2/SS//O-SnS/SS ASSCs device are calculated using eq 7 and 8 and plotted in Ragone plot as shown in Figure 11. The performance of ASSCs device in this work compared with previous work (Table 2) as CuO@ MnO2//MEGO (22 Wh kg−1, 85.6 kWh kg−1),46 rGO/MnO2/ CB//rGO/CB (20 Wh kg−1, 200 W kg−1),47 Si-diatom@ MnO2//AGO (23.2 Wh kg−1, 105 W kg−1),48 MnO2//AC (15.84 Wh kg−1, 885 W kg−1),49 PANI//Carbon Maxsorb (11.46 Wh kg−1),50 PPy//Carbon Maxsorb (7.64 Wh kg−1),50 PEDOT//Carbon Maxsorb (3.82 Wh kg−1),50 MnO2//PANI (5.86 Wh kg−1),50 MnO2//PPy (7.37 Wh kg−1),50 MnO2// PEDOT (13.37 Wh kg−1),50 MnO2//Fe3O4 (8.1 Wh kg−1),51 MnO2//Carbon (17.3 Wh kg−1),51 MnO2//AC (17.1 Wh kg−1,100 W kg−1),52 MnO2//AC (10 Wh kg−1),53 GNCC//AC (19.5 Wh kg−1),54 CNTs@NCS@MnO2//AC (27.3 Wh kg−1, 220 Wkg−1),55 GO/PPy//AC (20 Wh kg−1, 0.4 kWkg−1),56

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CONCLUSIONS A unique electrode design, specifically, porous β-MnO2 nanospheres and O-SnS nanoflowers on flexible stainless steel substrates exhibits a high specific capacitance of 994 and 1203 Fg1− at 5 mV s−1, respectively. Asymmetric all-solid-state supercapacitors (β-MnO2/SS//O-SnS/SS) fabricated from two optimum electrodes reach Cs of 122 Fg1− (at 5 mV s−1) show performance much higher than previous asymmetric supercapacitors based on MnO2 positive electrodes. The ASSCs 797


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Figure 10. (A and B) Initial and last GCD cycles. (C) Capacity retention versus cycle number plot (inset shows overlapped GCD cycles for first and 5000th cycle). (D and E) 3D Nyquist and 2D magnified Nyquist plot. (F) Bode plot of β-MnO2/SS//O-SnS/SS ASSCs device.

novel materials (β-MnO2 and O-SnS) is promising for future high energy density storage SCs.

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EXPERIMENTAL SECTION MATERIALS

All chemicals were of analytical grade and used as is. Potassium permanganate (KMnO4·6H2O), methanol, tin chloride (SnCl2), triethylamine (TEA), hydrochloric acid (HCl), and sodium thiosulfate pentahydrate (Na2S2O3·5H2O) were used without further purification (Sigma-Aldrich). The commercially available flexible stainless steel (SS) substrates (304 grade) were used as the current collector material. All solutions were prepared in double-distilled water (DDW). Preparation oF β-MnO2/SS Positive Electrode. The MnO2 thin films were prepared by the CBD method with some modifications. For preparation of MnO2 thin films, KMnO4 was used as the precursor, while methanol was used as the reducing agent. The matrix solutions were prepared by dissolving 0.1 M KMnO4 in 100 mL of DDW. The prepared solution was stirred for 10 min at room temperature to make a homogeneous distribution of the KMnO4 reagent. Furthermore, 4 mL of methanol was added in the above solution and the whole solution was added in a beaker containing vertically immersed SS substrate. This bath was kept at room temperature for 12 h. After that, the blackish brown colored β-MnO2/SS electrode was taken from the bath, washed in DDW, and dried at room temperature. The MnO2 content on the SS substrate was calculated by weighing the SS substrate before and after coating. Preparation of O-SnS/SS Negative Electrode. In a typical synthesis, SnCl2 and Na2S2O3 were used as tin and sulfur sources, respectively, while TEA was used as a complexing agent. The cationic source of 0.1 M SnCl2 was dissolved in DDW by continuous stirring using a magnetic stirrer. Three beakers of 50 mL were used for deposition of thin films at different time periods of 120, 240, and 360

Figure 11. Comparative Ragone plot of β-MnO2/SS//O-SnS/SS ASSCs device with literature (inset shows demonstration of ASSCs with four red, four yellow, and one yellow, and one green LEDs).

device has excellent cycling performance (stable at 95.3% after only 5000 GCD cycles, 10 mA), which denotes the excellent ability of the supercapacitor device. The large capacitance and high single ASSCs device voltage of +1.6 V allow high ED and PD of 29.8 and 1.3 kW kg−1, respectively. Additionally, a series combination of two ASSCs devices (+3.2 V) glows four yellow, four red, one yellow, and one green LEDs up to 2, 3, and 4 min, confirming the storage ability of the SCs device. In conclusion, the ASSCs device fabricated using the first combination of 798


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Table 2. Comparison of Electrochemical Performances of β-MnO2/SS//O-SnS/SS ASSCs Device in This Work with Those Reported in the Literature Sr. No

Supercapacitor device

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

CuO@MnO2//MEGO rGO/MnO2/CB//rGO/CB Sidiatom@MnO2//AGO MnO2//AC PANI//Carbon Maxsorb PPy//Carbon Maxsorb PEDOT//Carbon Maxsorb MnO2//PANI MnO2//PPy MnO2//PEDOT MnO2//Fe3O4 MnO2//Carbon MnO2//AC MnO2//AC GNCC//AC CNTs@NCS@MnO2//AC GO/PPy//AC

Na2SO4 Na2SO4 PVDF-Na2SO4 Na2SO4 KNO3 KNO3 KNO3 KNO3 KNO3 KNO3 K2SO4 K2SO4 Na2SO4 K2SO4 KOH Na2SO4 LiCl/PVA

18 19 20

MnO2//GO CNT/MnO2/GR ZnO@MnO2//RGO

Na2SO4 Na2SO4/PVP gel LiCl/PVA gel

21

CoSe2//MnO2

LiCl/PVA gel

22 23 24

SWCNTs//RuO2 Carbon aerogel//Co3O4 Graphene(ILCMG)// RuO2−IL-CMG Carbon cloth@T-Nb2O5@ MnO2//GO β-MnO2/SS//O-SnS/SS

25 26

Electrolyte

Specific capacitance (Cs)

Power density (kW kg−1)

Electrochemical stability (%) (cycles)

ref

22.1 24.3 23.2 15.84 11.46 7.64 3.82 5.86 7.37 13.5 8.1 17.3 17.1 10 19.5 27.3 20

85.6 21 0.105 0.885 45.6 48.3 54.1 42.1 62.8 120 10.2 19 0.1 16 5.6 220 0.4

101.5 after 10000 89 after 1000 84.8 after 2000 88 after 1000 − − − − − − − − 94 after 2000 77 102 after 10000 92.7 after 4000 86 after 2000

46 47 48 49 50 50 50 50 50 50 51 51 52 53 54 55 56

6.9 24.8 0.23 mWh cm−3

0.7 1.2 0.133 Wcm-3

93 after 5000 − 98.5 after 5000

57 58 59

0.58 mWh cm−3

0.282 Wcm−3

94.8 after 2000

60

H3PO4/PVA gel KOH-PVA gel H2SO4/PVA gel

49.2 F g−1at 0.25 A g−1 209 65.1 F g−1 at 0.5 A g−1 − − − − − − − 21.5 31 269 21 288 312.5 189.5 mF cm−2 at 10 mA cm−2 29.8 − 0.52 F cm−3 at 10 mV s−1 1.77 F cm−3 at 1 mA cm−2 138 F g−1 at 8 A g−1 57.4 F g−1 1 Ag1− 175 F g−1 at 0.5 A g−1

18.8 17.9 19.7

96 0.750 6.8

60−70 after 1000 85 after 1000 −̵

61 62 63

1 M Na2SO4

319 F g−1at 0.2 A g−1

31.76

2.25

85.7 after 2000

66

PVA-LiClO4

122 at 5 mV s−1

29.8

1.25

95.3 after 5000

this work

Characterization of β-MnO2 and O-SnS Thin Films. The structural study of the thin films was carried out by the XRD technique using the Bruker AXS D8 advance model with copper radiation (Cu Kα, λ = 1.54 Å). The chemical bonds existing in the film materials were precisely inspected using Fourier transform infrared (FTIR) spectroscopy. Field emission scanning electron microscopy (FE-SEM), transmission electron microscopy (TEM), and high resolution TEM (HRTEM) were used to scrutinize the surface morphology and nanostructure of films. Energy dispersed X-ray spectroscopy (EDS) were completed on the same FE-SEM microscope. The Rame-Hart instrument was used for contact angle measurements. X-ray photoelectron spectroscopy (XPS) analysis was carried out for finding the chemical composition and oxidation states of the film materials using an ESCALAB 250-Xi X-ray photoelectron spectrometer microprobe. SSA and porosity were studied by the Brunauer−Emmett−Teller (BET) analysis, and the pore size distribution was determined from the Barrett−Joyner−Halenda (BJH) method. The electrochemical supercapacitive measurements were carried out by an automatic battery cycler WBCS-3000 unit in a three-electrode system of β-MnO2 and O-SnS thin film on an SS substrate as a working electrode, saturated calomel electrode (SCE) as a reference electrode, and platinum as a counter electrode. The areas of β-MnO2 and O-SnS on each electrode dipped into electrolyte were 1 cm2. The electrochemical impedance study was carried out by an electrochemical workstation ZIVE MP1 unit within a frequency range of 10 kHz to 100 mHz through an ac amplitude of 10 mV. Similar instruments were used for investigation of capacitive performance of the β-MnO2/SS//O-SnS/SS ASSCs device using a two-electrode system.

min. Afterward, SnCl2 was dissolved in DDW, and 1 mL of TEA was mixed dropwise in each beaker solution. Then, 0.15 M of Na2S2O3 was added in the above solution. Likewise, dilute 1 M HCl was added dropwise in solution until pH becomes 4 (±0.1). These above three beakers were placed in a constant temperature bath, which was maintained at a temperature of 343 K. The deposition of SnS thin films was carried out at the deposition times of 120, 240, and 360 min to alter the surface morphology of thin film, and deposited electrodes were denoted as TS1, TS2, and TS3, respectively. Assembly OF β-MnO2/SS//O-SnS/SS ASSCs Device. The ASSCs device was assembled using β-MnO2 as the cathode electrode and O-SnS as the anode electrode using a PVA-LiClO4 gel electrolyte membrane between electrodes. The solid PVA-LiClO4 gel electrolyte was prepared by mixing 6 g of PVA powder, 2 g of LiClO4, and 60 mL of DDW, heating all to 70 °C under a vigorous stir until the solution became clear and then slowing the stir for 6 h at room temperature in order to eliminate the bubbles engendered during the dissolution. Also, to maintain q+ = q−, the weight ratio (1:1) between β-MnO2/SS and O-SnS/SS was adjusted using following formula:24

M1 C1 × V 1 = M2 C2 × V 2

Energy density (Wh kg−1)

(10)

where M1 and M2, C1 and C2, and V1 and V2 are the masses, Cs, and potential windows of the β-MnO2 positive electrode and O-SnS negative electrode (TS2), respectively. The size of the ASSCs device was about 5 cm × 5 cm. After packing of the ASSCs device, it is placed in a hydraulic press carrying pressure of 1 ton for 24 h. The lower thickness (450 μm) of the ASSCs device facilitates a close contact between the electrolyte and the electrode. 799


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(11) Li, Y.; Tu, J. P.; Huang, X. H.; Wu, H. M.; Yuan, Y. F. Nanoscale SnS with and without Carbon-Coatings as an Anode Material for Lithium Ion Batteries. Electrochim. Acta 2006, 52, 1383−1389. (12) Xu, Y.; Al-Salim, N.; Bumby, C. W.; Tilley, R. D. Synthesis of SnS Quantum Dots. J. Am. Chem. Soc. 2009, 131, 15990−15991. (13) Liu, Y.; Hou, D.; Wang, G. Synthesis and Characterization of SnS Nanowires in Cetyltrimethylammoniumbromide (CTAB) Aqueous Solution. Chem. Phys. Lett. 2003, 379, 67−73. (14) Koktysh, D. S.; McBride, J. S.; Rosenthal, S. J. Synthesis of SnS Nanocrystals by the Solvothermal Decomposition of a Single Source Precursor. Nanoscale Res. Lett. 2007, 2, 144−148. (15) Gao, H.; Xiao, F.; Ching, C. B.; Duan, H. High-Performance Asymmetric Supercapacitor Based on Graphene Hydrogel and Nanostructured MnO2. ACS Appl. Mater. Interfaces 2012, 4, 2801− 2810. (16) Zhao, Y.; Ran, W.; He, J.; Huang, Y.; Liu, Z.; Liu, W.; Tang, Y.; Zhang, L.; Gao, D.; Gao, F. High-Performance Asymmetric Supercapacitors Based on Multilayer MnO2/Graphene Oxide Nanoflakes and Hierarchical Porous Carbon with Enhanced Cycling Stability. Small 2015, 11, 1310−1319. (17) Chodankar, N. R.; Dubal, D. P.; Lokhande, A. C.; Kim, J. H.; Lokhande, C. D.; Patil, A. M. An Innovative Concept of Use of Redoxactive Electrolyte in Asymmetric Capacitor Based on MWCNTs/ MnO2 and Fe2O3 Thin Films. Sci. Rep. 2016, 6, 39205−39218. (18) Zhang, Z.; Chi, K.; Xiao, F.; Wang, S. Advanced Solid-State Asymmetric Supercapacitors Based on 3D Graphene/MnO2 and Graphene/Polypyrrole Hybrid Architectures. J. Mater. Chem. A 2015, 3, 12828−12835. (19) Yang, J.; Li, G.; Pan, Z.; Liu, M.; Hou, Y.; Xu, Y.; Deng, H.; Sheng, L.; Zhao, X.; Qiu, Y.; Zhang, Y. All-Solid-State High-Energy Asymmetric Supercapacitors Enabled by Three Dimensional MixedValent MnOx Nanospike and Graphene Electrodes. ACS Appl. Mater. Interfaces 2015, 7, 22172−22180. (20) Ghosh, K.; Yue, C. Y.; Sk, M. M.; Jena, R. K. Development of 3D Urchin-Shaped Coaxial Manganese Dioxide@Polyaniline (MnO2@PANI) Composite and Self-Assembled 3D Pillared Graphene Foam for Asymmetric All-Solid-State Flexible Supercapacitor Application. ACS Appl. Mater. Interfaces 2017, 9 (18), 15350−15363. (21) Ma, Z.; Shao, G.; Fan, Y.; Feng, M.; Shen, D.; Wang, H. Fabrication of High-Performance All-Solid-State Asymmetric Supercapacitors Based on Stable α-MnO2@NiCo2O4 Core−Shell Heterostructure and 3D-Nanocage N-Doped Porous Carbon. ACS Sustainable Chem. Eng. 2017, 5 (6), 4856−4868. (22) Wang, J.; Liu, S.; Zhang, X.; Liu, X.; Liu, X.; Li, N.; Zhao, J.; Li, Y. A High Energy Asymmetric Supercapacitor Based on Flower-like CoMoO4/MnO2 Heterostructures and Activated Carbon. Electrochim. Acta 2016, 213, 663−671. (23) Goubard-Bretesche, N.; Crosnier, O.; Buvat, G.; Favier, F.; Brousse, T. Electrochemical Study of Aqueous Asymmetric FeWO4/ MnO2 Supercapacitor. J. Power Sources 2016, 326, 695−701. (24) Chen, R.; Liu, L.; Zhou, J.; Hou, L.; Gao, F. High-Performance Nickel-Cobalt-Boron Material for an Asymmetric Supercapacitor with an Ultrahigh Energy Density. J. Power Sources 2017, 341, 75−82. (25) Chauhan, H.; Singh, M. K.; Kumar, P.; Hashmi, S. A.; Deka, S. Development of SnS2/RGO Nanosheets Composite for Cost-Effective Aqueous Hybrid Supercapacitors. Nanotechnology 2017, 28, 025401− 025412. (26) Wang, X.; Li, Y. Selected-Control Hydrothermal Synthesis of αand β-MnO2 Single Crystal Nanowires. J. Am. Chem. Soc. 2002, 124, 2880−2881. (27) Chodankar, N. R.; Gund, G. S.; Dubal, D. P.; Lokhande, C. D. Alcohol Mediated Growth of α-MnO2 Thin Films from KMnO4 Precursor for High Performance Supercapacitors. RSC Adv. 2014, 4, 61503−61513. (28) Radhakrishnan, S.; Kim, H. Y.; Kim, B. S. A Novel CuS Microflower Superstructure Based Sensitive and Selective Nonenzymatic Glucose Detection. Sens. Actuators, B 2016, 233, 93−99. (29) Yuan, L.; Lu, X. H.; Xiao, X.; Zhai, T.; Dai, J.; Zhang, F.; Hu, B.; Wang, X.; Gong, L.; Chen, J.; Hu, C.; Tong, Y.; Zhou, J.; Wang, Z. L.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.7b03136. FTIR spectrum and TEM image of β-MnO2 thin film; CV plots; GCD plots of β-MnO2 electrode; Cs versus scan rate plot; Cs versus current density plot; Bode plots of β-MnO2 electrode; CV plots of TS1, TS2, TS3, and SS substrates; Cs versus scan rate plots (histograms of Cs for TS1, TS2, and TS3 electrodes); GCD plots of TS1, TS2, and TS3 electrodes; Cs versus current density plots of TS1, TS2, and TS3 electrodes. (PDF)

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AUTHOR INFORMATION

Corresponding Author

*Tel: +91 231 2601212. Fax: +91 231 2601595. E-mail:- l_ [email protected]. ORCID

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

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors are grateful to the Department of Science and Technology-Science and Engineering Research Board (DSTSERB), New Delhi, India, for their financial support through research project no. SERB/F/7448/2016-17 dated 13 January, 2017; DST-INSPIRE research project [DST/INSPIRE/04/ 2016/000260].

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NOTE ADDED AFTER ASAP PUBLICATION This article was published ASAP on December 11, 2017, with an incorrect version of Scheme 2. The corrected article was published ASAP on January 2, 2018.

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High Performance All-Solid-State Asymmetric Supercapacitor Device

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