High Performance All-Solid-State Asymmetric Supercapacitor Device

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High Performance All-Solid-State Asymmetric Supercapacitor Device Based on 3D Nanospheres of #-MnO2 and Nanoflowers of O-SnS Amar Patil, Vaibhav Lokhande, Umakant Patil, Pragati Shinde, and Chandrakant D. Lokhande ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.7b03136 • Publication Date (Web): 22 Nov 2017 Downloaded from http://pubs.acs.org on November 22, 2017

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ACS Sustainable Chemistry & Engineering

High Performance All-Solid-State Asymmetric Supercapacitor Device Based on 3D Nanospheres of β-MnO2 and Nanoflowers of O-SnS Amar. M. Patila, Vaibhav. C. Lokhandeb, Umakant. M. Patila, Pragati. A. Shindea, Chandrakant. D. Lokhandea* a

Centre for Interdisciplinary Research, D. Y. Patil University, Kasaba Bavada, Kolhapur, Maharashtra 416006 (MS), India

b

Department of Electronics and computer Engineering, Chonnam National University, Gwangju 500-757, South Korea.

AUTHORS 1. Amar M. Patil E-mail:[email protected] 2. Vaibhav C. Lokhande E-mail:[email protected] 3. Umakant M Patil E-mail:[email protected] 4. Pragati. A. Shinde E-mail:[email protected] 5. Chandrakant D. Lokhande E-mail:- [email protected] CORRESPONDING AUTHOR *Prof. Chandrakant. D. Lokhande Centre for Interdisciplinary Research, D. Y. Patil University, Kasaba Bavada, Kolhapur, Maharashtra 416006 (MS), India Tel: +91 231 2601212, Fax: +91 231 2601595 E-mail:- [email protected]

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ABSTRACT The current problem of the relatively low energy density and cycling stability of supercapacitors can be effectively addressed by designing all-solid-state high performance asymmetric supercapacitor (ASSCs) using chemically deposited beta-manganese oxide (βMnO2) as a positive active electrode and orthorhombic-tin sulfide (O-SnS) as a negative electrode material on stainless steel (SS) substrate with polyvinyl 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 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, energy and power densities (ED and PD) of 83.3, 69.2 Wh kg-1 and 10 and 1.8 kWh kg-1, respectively. Fabricated ASSCs device exhibits Cs of 122 F g-1, ED of 29.8 Wh kg-1, 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 charge.

KEYWORDS β-MnO2;

O-SnS;

Thin

films;

Nanostructure;

Electrochemical

stability;

Asymmetric

supercapacitor;


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INTRODUCTION On the way to come across the growing energy demands of society for human life activities and for developed electronic technology, it is essential to accumulate extreme 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 and storage devices. Among all, supercapacitors (SCs) are promising for energy storage because of its high power density (PD), long cycling life, and easy fabrication as well as safe operation1,2, even though they generally have lower energy density (ED) (less than 10 Wh kg-1) than ordinary batteries3. The strategy to reach a high ED and longer cycling life is to emerging innovative electrode materials, which have high specific capacitances (Cs) at its wide operating potentials windows. Transition metal oxides (pseudocapacitive) such as MnO24 have been furthermost extensively studied as a positive electrode owing to its low cost, ecofriendly nature, high theoretical Cs and positive wide potential window (more than + 0.8 V). Synthesis of nanostructure of MnO2 thin films on stainless steel (SS) substrate is easy, uniform and highly adherent which helps to enhance cycling performance of electrode5. Sn-based metal chalcogenide materials are used for different applications such as SCs, lithium ion batteries and solar cell 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 the large volume extension. On the other side, in order to choose negative electrode material, noticing the limitations of easily available and low cost negative carbon electrodes (carbon aerogel, activated carbon and graphite electrodes) such as their high resistivity owing to contact resistance between carbon particles, causing rise in internal series resistance, which is leading to decrement in electrochemical performance of negative electrode6. Stannous


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sulfide (SnS) is immersing as a fascinating new electrode material for electrochemical capacitors. SnS and SnS2 are progressively significant due to their especially semiconducting properties7. They also have substantially high chemical stability; thus eludes corrosion in SCs device. Additionally, Sn and S have fairly little toxicity and not dangerous to environment. The nontoxicity and abundant availability in nature supports to the fabrication of SCs devices that are ecologically safe8,9. The properties like high adsorption coefficient, wide operating potential window (> - 0.7 V/SCE) and high storage capacity with high SSA fix the scope of SnS electrode in future SCs. The electrochemical supercapacitive performance of SnS electrode material can be further improved by increasing electric conductivity, porous surface and potential windows of electrode9. 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 co-heated with polyvinyl alcohol at different temperatures to get carbon coating. Xu et al.12 prepared SnS nanocrystals using ethanolamine ligands. Liu et al.13 synthesized single crystalline SnS nanowires using cationic surfactant cetyltrimethylammonium bromide (CTAB). Koktysh et al.14 prepared SnS nanocrystals (NCs) from bis (diethyldithiocarbamato) tin (II) in oleylamine at higher temperature. The ED, which 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 as using organic electrolytes and fabricating asymmetric supercapacitor15. Organic electrolytes can offer an improved electrochemical stability, but suffer from lower ionic conductivity and toxicity16. Therefore, asymmetric solid state supercapacitor (ASSCs) design in gel electrolytes is an effective approach to enhance the operating potential window and afford maximum ED. In the efforts to enhance ED, MnO2 based


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ASSCs such as MWCNT/MnO2//Fe2O317,Ni/GF/MnO2//Ni/GF/Ppy18,Al@Ni@MnOx//GF19, MnO2@PANI//GF20, α-MnO2@NiCo2O421, CoMoO4/MnO222, FeWO4/MnO223, Ni-Co-B//CAC24 and SnS based ASSCs as SnS2/RGO25 have been reported previously. On the other hand, carbon electrode shows lower Cs and high cost, which discards the possibility of carbon material as a negative electrode compared with SnS electrode. Therefore, it is reasonable to assume that βMnO2/SS//O-SnS/SS ASSCs would possess more excellent electrochemical performance than that of β-MnO2 and O-SnS based other ASSCs devices. Till now, no reports are available for fabrication of high performance SS/β-MnO2//O-SnS/SS ASSCs device. Present work, succeeds in coalescing all the discussed advantages through the preparation of MnO2-nanospheres and SnS–nanoflowers nanostructured binder-free electrodes for ASSCs directly on cost efficient, high mechanical strength, excellent flexible and stable in acidic electrolyte SS substrate. Our strategy is to develop ASSCs device to achieve a wide potential window, longer cycling life and higher energy storage SCs using above discussed positive (βMnO2, Optimizing results in a Cs of 994 Fg-1 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 Fg-1 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 by PVA-LiClO4 polymer gel electrolyte. After completion of initial basic characterizations of electrodes, electrochemical properties are tested using battery cycler and electrochemical workstation units. Series connected two ASSCs devices charged for the 30 s and 4 yellow, 4 red and 1 yellow and 1 green LEDs used for demonstration of application of ASSCs RESULTS AND DISCUSSION The synthesis process of positive β‒MnO2 and negative O-SnS electrodes are shown in the scheme 1. The preparation of thin films with steps like nucleation, aggregation, coalescence and


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growth is displayed schematically. The reaction mechanisms of synthesis of films are explained with drawing bond diagrams. Figure 1(A) displays the XRD pattern of chemical bath deposited (CBD) MnO2 thin film. The observed diffraction peaks corresponds to (110), (211) and (002) planes, accord with the standard characteristic diffractions of β-type tetragonal MnO2 (JCPDS card no. 24‒0735)26. No other diffraction peaks observed than SS substrate peaks resembles to the pure phase formation of β‒MnO2. The wide and low intense diffraction peaks denotes nanocrystalline nature of β‒MnO2, which beneficial for SCs application27. This structure offers rapid ion diffusion in electro-active material, which increase the charge storing capacity of the SCs. The peaks marked by an asterisk (*) are corresponding to the SS substrate.

Scheme 1 (A) Synthesis process of β-MnO2 and O-SnS nanostructures with chemical bath deposition method, the deposition steps indicate nucleation, aggregation, coalescence and growth of thin film and (B) the bond structure alteration during synthesis of thin film electrodes. FTIR spectrum show the intense peak at 3354 cm-1, which is related to –OH stretching vibrations of MnO2 (Figure S1, Electronic Supporting Information (ESI)). The absorption peaks


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at 1640 cm-1 and 1405 cm-1 are accredited to –C-O bending vibrations joint with Mn atoms. The absorption peak at 615 cm-1 accompanying with the coupling mode among Mn-O stretching modes28.

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, and (F) BJH pore size distribution plot of β-MnO2 powder sample. Additionally, complete information about chemical compositions and electronic states of β-MnO2 is investigated through XPS study. Figure 1(B) shows the survey spectrum of β-MnO2 thin film material. From vigilant evaluation of the XPS survey spectrum, it demonstrates 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 hydrocarbon from the XPS machine29. The Mn 2p peak can be divided into the 2p3/2 and 2p1/2 peaks at binding energy of 642.3 and


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653.9 eV (see Figure 1(C)), respectively, denotes that the chemical state of element Mn is in Mn4+ state30. The separation of energy (11.8 eV) between the two states of Mn 2p3/2 and Mn 2p1/2 is well accordance with the literature31, suggesting the β-MnO2 is developed on SS substrate. Figure 1(D) represents an XPS spectrum of O 1s showing peaks of O 1s at 529.68 and 531.3 eV is allocated to Mn-O-Mn and Mn-O-H, respectively32. The nitrogen adsorption-desorption isotherms and corresponding pore size distribution plot of 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 MnO2 electrode material. The BET SSA of the β-MnO2 nanostructures is about 75 m2 g-1 and pores are distributed overall on surface with pore radius ranging from 1.83 to 12.39 nm, confirms meso/macro-porous range, which is supportive to storage of charge in porous interfaces.

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


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The surface morphology investigations of β-MnO2 thin film on SS substrate are carried out through FE-SEM analysis. Figure 2(A, B and C) shows FE-SEM images of β-MnO2 thin film at magnifications of 10,000 X and 25,000 X and 50,000 X, respectively. The spherical shaped βMnO2 nanoparticles are distributed overall on the substrate surface. The high resolution transmission electron micrographs of these nanospheres are shown in Figure S2 (ESI). Single nanosphere is shown in Figure 2(C), and average diameter of this single nanosphere is about 550 nm. The average size of overall nanospheres grown on film surface is about 350 nm. The nanospheres are distributed overall on the surface of SS substrate showing different size of nanospheres. The contribution of electroactive material in 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 nanostructure33. The elemental mapping of Mn and O elements are depicted in Figure 2(D and E), respectively. The Figure 2(F) shows energy dispersive X-ray spectrum for the βMnO2 thin film. In the spectrum, Mn and O element peaks are originated. Some peaks other than Mn and O are observed due to SS substrate. It reveals that the electrode consists of Mn and O elements, confirms the formation of MnO2 on SS substrate. In order to prepare best performance negative electrode, three different nanostructures of SnS thin films are achieved basically by varying the deposition time. The Sn ions are released from 0.1 M SnCl2 solution and form complex with triethylamine (TEA) (Equation 1 and 2). The Na2S2O3 gives H2S in double distilled water (DDW) (Equation 3). The complex of Sn+2 ions with TEA and H2S after addition of HCl gives SnS thin films on a SS substrate at 343 K (Equation 4) (Scheme 1)


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. 2 → 3 1 →

2

→ (3)

→ ↓ ↓ ↓ 2 ↑ 4

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, The XPS spectra of TS2 thin film of (C) survey scan spectrum, (D) Sn 3d core level spectrum and (E) S 2p core level spectrum, and (F) The N2 adsorption-desorption isotherms of (a) TS1, (b) TS2 and (c) TS3 thin films prepared at different deposition time periods, and inset show pore size distribution plot of TS2 powder sample.


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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 high intense peak, 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 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 lacking of impurity peaks designates that the O-SnS thin films has high crystallinity34. The SS substrate peak is denoted by ‘∆’. FTIR spectra of TS1, TS2 and TS3 thin films are displayed in Figure 3(B), peaks at wave numbers of 3750, 1606, 1350, 832 and 640 cm-1 support to the formation of SnS material on SS substrate. The peaks observed at 640 and 832 cm−1 in the spectrum are due to the presence of Sn–S bonds35. The two smaller peaks at 1350 and 1606 cm−1 are owing to the formation of C–O and C–H bonds, respectively. The peak appeared at 3750 cm-1 is associated with stretching mode, which 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 remarks a number of functional groups present in adherence O-SnS thin films. The wide scan XPS spectrum of O-SnS thin film prepared at the deposition time of 240 min is shown in Figure 3(C). The spectrum showed peaks that were assigned to Sn 3d3/2, Sn 3d5/2, S 2p3/2, C 1s and O 1s states. The occurrence of 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


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two main peaks corresponding to Sn 3d5/2 and Sn 3d3/2 that are positioned at 486.5 eV and 495 eV, respectively. The energy separation among these two peaks is about ~ 8.5 eV, which well in covenants with that reported in the literature36. The observed binding energies of the two peaks are dissimilar from that of those values of elemental Sn in SnO2, SnS2 and Sn2S3, this designates that Sn in SnS phase and +2 oxidation states. In addition to these main peaks, S 2p3/2 state is observed at 168.6 eV (Figure 3(E)). This confirmed the occurrence of only O-SnS phase in the film layers.

(013) 0.26 nm

Figure 4. The 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, and (G) TEM image of TS2 powder sample scratched from TS2 film (Inset shows HRTEM image of TS2 film).


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The higher SSA and suitable pore volume size of synthesized electrode materials are important for higher electrochemical performance, which delivers the larger electroactive sites in electrochemical reactions and suitable pore volume offers an

easy way for an

intercalation/deintercalation reaction. N2 adsorption-desorption isotherm of TS1, TS2 and TS3 powder samples are portrayed in Figure 3(F). The each isotherm curve of TS1, TS2 and TS3 samples reveal typical type III isotherm with H3 hysteresis loop37. The observed BET SSA of 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 inset, peak positions at 3.05, 5.5, 12.3, 19.6, 23.4 and 33.5 nm in the pore size distribution graph depicts the meso/macro-porous range of the pores. Chauhan etal.7 reported synthesis of tin sulfide nanoparticles using the solvothermal route for applications in SCs devices. The nanoflowers of TS2 sample surface show higher SSA with a smaller pore size of 3.05 nm. The better SSA of TS2 electrode is helpful for more contribution of electro-active electrode material in electrochemical reactions. The peak positions between 3 to 34 nm in the pore size distribution plot designate that the dissimilar sized meso/macro-porous are available in electrode material. Figure 4(A-C) depicts the surface morphologies of TS1, TS2 and TS3 thin films at magnifications of 10,000 X, which show 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. TS1 thin film shows the formation of a large number of regularly spread nanoflakes with rough and inferior porous structure (see Figure 4(A)). At deposition time of 240 min, 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


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nanoflakes reduces with formation of nanoflowers (each has size about 563 nm), which increase the porous surface for TS2. Furthermore, as deposition time increases, the growth rate of reaction increases with the nucleation rate and surface converts inactive with overgrowth of material on a 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 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 nanoflowers shaped surface offer more SSA in electrochemical reactions38. Nanoflowers of TS2 film maximizes the electro-active materials utilization ratio and drops the diffusion length significantly cause in 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 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. TS2 electrode stretches hydrophilic nature as it shows the lower contact angle useful to improve interaction of the electrode with electrolyte for resultant higher SCs performance of the electrode. Figure 4(D and E) depicts corresponding elemental analysis of TS2 illustrates Sn and S elements with an atomic ratio near to (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 in near same percentage (see Figure 4(F)). There are some very


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small impurity peaks, which are ascribed to the SS substrate and air contamination of thin film. The TEM images of TS2 nanoflowers are described in Figure 4 (G), and HRTEM image of TS2 nanostructure is shown in the inset, the lattice fringes can be visibly observed with a d-spacing of 0.26 nm, corresponding to the (013) plane of O-SnS.

l

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 and, (D) enlarged Nyquist plot of β-MnO2 electrode (inset shows equivalent circuit from which Nyquist plot is generated). The Cs of β-MnO2, O-SnS and β-MnO2/SS//O-SnS/SS ASSCs device from scan rate is calculated using the following equation,


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,-

1 Cs = ) I V dV 5 ms V& − V( ,.

Where, ‘Cs’ is specific capacitance, ‘m’ is mass on 1 cm2 area, ‘s’ is scan rate, (V1 – V0) is potential window and ‘I’ is current response. The Cs values are deliberated from GCD analysis using following equation as, C0 =

1×3 6 4 × ∆6

The charge storage and charge-discharge capacity of electrode materials are analyzed with ED and PD. These values are calculated using below relations, 6& − 6( × 0.5 × 0 8 = , ; 7 3.6 =8 =

3600 × 8. 8 ;3

Where, V1-V0 is potential window and Cs is specific capacitance (F g-1). Where, ‘I’ is the current density, ‘m’ is a mass of material loaded at 1 cm2, ‘t’ is discharging time, and ‘V’ is potential window. In order to fabricate efficient ASSCs device, the electrode selection is a primary stage. The ideal electrochemical properties of β-MnO2 electrode material prove it is a promising positive electrode in ASSCs device. The electrochemical SCs properties of β-MnO2 electrode are examined using the CV, GCD and EIS measurements in 1 M Na2SO4 electrolyte. The ideal SCs behaviors such as, rectangular shape of CV curve, symmetry in both anodic and cathode sides of CV curve and higher area under the CV curve is accomplished by as prepared β-MnO2 electrode (Figure S3(A), ESI). These properties of electrode confirm the storage of electric energy is mostly pseudocapacitive39. As scan rate increases, the area under CV curve also increases, which designate effective consumption of active β-MnO2 material with electrolyte ions in SCs. GCD


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analysis of electrode for different current densities of 2, 3, 4 and 5 mA cm-2 (Figure S3(B), ESI), confirms the charge storage contribution comes from surface adsorption/desorption of electrolyte ions and intercalation/deintercalation of protons in electrode material means reversible redox reactions40. 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 β-MnO2 electrode (area of 1 cm ⨉ 1cm) (Figure S3(C and D), ESI). At lower scan rate and current density, the rate of intercalation/deintercalation reaction is longer, which may be attributed to maximum utilization of electro-active material in electrochemical reactions provide higher Cs as nanostructure of β-MnO2 electrode material facilitates easy way for intercalation/deintercalation of electrolyte ions by reducing diffusion resistance41. Figure 5(A) shows Ragone plot of β-MnO2 electrode material which displays highest ED and PD of 83 Wh kg-1 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 electrode contribute more material in electrochemical reaction from 50 to 250 GCD cycles, the stability increases up to 110.78% and afterwards it decreases up to 94.3 % at 3000 cycles (Figure 5(C)). The overlapping of GCD cycles on each other indicates better cycling stability of electrode material. The magnified Nyquist plots of β-MnO2 electrode material in 1 M Na2SO4 electrolyte are shown in Figure 5(D). Inset shows an equivalent circuit diagram fitted with Nyquist plot. The Nyquist plot analysis gives 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 electrode material42. The phase angle at a higher frequency range is inferior because


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of the ionic resistance of electrolyte corresponds to lower capacitance (Figure S3(E), ESI). Phase angle increases toward -90° at the lower frequency range denote capacitive behavior of β-MnO2 electrode. Overall performance of β-MnO2 electrode shows promising positive electrode material for the ASSCs supercapacitor device. On the other side, TS1, TS2 and TS3 electrodes have been employed to CV, GCD and EIS tests examine the influence of deposition time on the electrochemical supercapacitor performance of negative electrode. CV plots of bare SS, TS1, TS2 and TS3 electrodes within potential range of -0.5 to -1.2 V/SCE in 2 M KOH electrolyte at a scan rate of 100 mV s-1 are shown in Figure S4(A) (ESI). The current densities of TS1, TS2 and TS3 electrodes are greater than the bare SS substrate, indicating that the contribution of SS substrate to the Cs of O-SnS is negligible. The area under the CV curve increases with increase in deposition time up to 240 min. The performance achieved is dependable with the morphologies, signifying the nanoflowerlike morphology offer more SSA fascinates fast charge intercalation/deintercalation reactions for TS2 electrode. The CV curves of TS2 electrode in the scan rates of 5, 10, 50 and 100 mV s-1 are depicted in Figure S4(B) (ESI). The CV curves display rectangular shape with small noticeable redox peaks at the potential positions of -0.6 and -1.0 V/SCE indicates that the charge storage is due to redox reactions as well as EDLC type reactions. The kinetic irreversibility of electrode shows asymmetry on the oxidation and reduction sides of CV curves43. The rise in area under the CV curve with scan rate is clearly observed for each electrode. The intercalation of OH- ions of KOH electrolyte to the surface of O-SnS carried out at charging and the similar deintercalation takes place during discharging. The charge transfer in O-SnS electrode material is based on following reaction, SnS OH. ↔ SnSOH . e . 9


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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) (ESI) 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 lower scan rate than high scan rate. On the opposite side, at higher scan rate only surface of electrode material contributes in the electrochemical reaction not inside of material, which provides a lower Cs.

I

Figure 6. (A) Ragone plots and (B) The 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, and (D) Nyquist plots of TS1, TS2 and TS3 electrodes (Inset shows best fitted equivalent circuit).


19


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The GCD curves at an identical current density of 1 mA cm-2 for TS1, TS2 and TS3 electrodes which is synthesized at three different deposition time periods are shown in Figure S4(D), ESI). The higher discharge time is observed for 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 range from -1.2 to -0.5 V/SCE (see Figure S4(E), ESI), indicates the capacitance contributed not only from pseudocapacitance but also from EDLC type. The minimum interface resistance (IR) drop is detected for TS2 electrode as compared to TS3 and TS1 electrodes. The non-linear performance of charge-discharge curve designates the electrochemical adsorption and desorption reactions at the interface of electrode and electrolyte. The Cs values attained for 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), ESI), confirms discharging rate of the TS2 electrode is higher charge storage ability than other electrodes. Ragone plots of TS1, TS2 and TS3 electrodes are shown in Figure 6(A), exhibits 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 is relatively remarkable as compared to the earlier reports on SnS (1.49 Wh Kg-1 at 0.24 kW Kg-1) and SnS2/GO (16.67 Wh Kg-1 at 0.48 kW Kg-1)25. The electrochemical stability of 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 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 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


20

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impedance plot is fitted. The Nyquist plots of TS1, TS2 and TS3 electrodes show the semicircle at higher frequency region and intercept on the real axis stretches ESR of an electrode material, which contains electronic and ionic contributions44,45. 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 brings low impedance and provide easy access to electrolyte ions for intercalation and deintercalation. The diameter of semicircle gives Rct of 3.88, 1.18 and 1.85 Ω cm-2 for TS1, TS2 and TS3 electrodes, respectively. The diffusion of Sn2+ in electrolyte gives a straight line after semicircle at 45°, which describes the Warburg’s constant (W). The TS2 electrode displays lower impedance than TS1 and TS3 electrodes. Hence, the nanoflowers morphological TS2 electrode is suitable for negative electrode in ASSCs device (see Table 1). Table 1 Electrochemical properties of positive and negative electrodes of ASSCs. Sr.

Sample

Morphology

Electrolyte

No

1

β-MnO2

Specific

Energy

Power

Capacity

Rct

ESR

Capacitan-

density

density

retention

Ωcm-2

Ωcm-2

ce (F g-1)

(W kg-1)

kW kg-1

(%)

83.3

10

94.3

3.9

0.62

10.5

86

3.88

0.85

Nanosphere

1M Na2SO4

994

2

TS1

Nanoflake

2 M KOH

950

58.8

3

TS2

Nanoflower

2 M KOH

1203

69.2

8.5

90.2

1.18

0.15

4

TS3

Nanogranule

2 M KOH

1055

60.5

10.1

88.6

1.85

0.46

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) showed the XRD patterns of the electrodes of βMnO2 and O-SnS, before and after cycling, respectively. The SS substrate peaks are denoted by ‘*’ asterisk. After cycling β-both of the electrodes shows all of the XRD peaks corresponding to


21


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β-MnO2 and O-SnS phase with slightly decreased peak intensity. The slight decrease in peak intensity is attributed to the electrochemical dissolution of β-MnO2 and O-SnS in Na2SO4 electrolyte. The present results clearly highlighted the significant structural stability of the both electrodes.

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) O-SnS electrode before and after cycling. Figure 7 (C and D) shows Nyquist plots of β-MnO2 and O-SnS 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 semicircle in the higher frequencies revealed the charge transfer resistance (Rct) of β-MnO2 and O-SnS electrodes


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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.

Scheme 2 Schematic representation of fabrication process of ASSCs device. After optimization of high performance electrodes, β-MnO2/SS//O-SnS/SS ASSCs device 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 denoting a combination of these two electrodes can be achieve a large potential as a asymmetric device (see Figure. 8(A)). The CV curves of 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 device shows the higher current response as well as the area under the CV curve with respect to potential, so +1.6 V potential is selected for further electrochemical investigations. CV curves at different scan


23


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rates ranging from 5 to 100 mV s-1 are shown in Figure 8(C). The plots of Cs versus scan rate and current for ASSCs device included in Figure 8(D), which showmaximum Cs of 122 F g-1 at 5 mV s-1 scan rate.

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, and (D) Cs versus scan rate plot of β-MnO2/SS//O-SnS/SS ASSCs and inset shows Cs versus current density plot of ASSCs device.


24

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Figure 9 (A) GCD curves of β-MnO2, O-SnS electrodes and β-MnO2/SS//O-SnS/SS ASSCs device at same current of 8 mA, (B) The GCD curves of ASSCs device 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 current (8, 10 and 12 mA) and (D) CV curves of ASSCs device at different bending angles at 100 mV s-1. The comparative GCD curves of β-MnO2, O-SnS electrodes and β-MnO2/SS //O-SnS/SS ASSCs device is depicted in Figure 9(A). The GCD curves of 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


25


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shown in Figure 9(B). The columbic efficiency of device at +1.6 V potential is high which confirms that this potential is suitable for β-MnO2/SS//O-SnS/SS ASSCs device. The GCD curves of 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 negligible effect (2 % Cs loss) of bending on area under the CV curve (see Figure 9(D)).

l

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, and (F) Bode plot of β-MnO2/SS//O-SnS/SS ASSCs device The electrochemical cycling stability of β-MnO2/SS//O-SnS/SS ASSCs device is examined by taking GCD curves up to 5000 cycles. Figure 10(A and B) shows few first and


26

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5000th GCD cycles. The shape of GCD curve remains equivalent for 1st and 5000th CV cycles indicate more capacity retention. The capacity retention with GCD cycle number plot shown in Figure 10(C) depicts capacity retention of 95.3 % is remains up to 5000 GCD cycles. Inset shows nearly overlapped 1st and 5000th GCD cycles taken at 10 mA. The achieved better electrochemical stability performance is owing to the capacity of electrode materials and used polymer gel electrolyte. Likewise, the EIS analysis of β-MnO2/SS//O-SnS/SS ASSCs device performed at open circuit potential (Figure 10(D and E)). The semicircle is observed at high frequency range and after that the straight line at the lower frequency range indicates Warburg’s resistance (W). The diameter of semicircle gives Rct of 0.24 Ω and first intercept on real x-axis gives Rs of 0.22 Ω. The lower values of resistance are beneficial for electrochemical performance of β-MnO2/SS//O-SnS/SS ASSCs device. The phase angle at a higher frequency range is lower due to internal resistance of electrolyte corresponds to lower capacitance and phase angle shifts toward -90° at the lower frequency range specifies capacitive behavior of β-MnO2/SS//O-SnS/SS ASSCs device (see Figure 10(F)). The ED and PD values of β-MnO2/SS//O-SnS/SS ASSCs device are calculated using equation 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 (see 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, Sidiatom@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 ,


27


Page 28 of 45

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 and other aqueous, solid state supercapacitor57. The maximum ED and PD of 29.8 Wh kg-1 and 1.25 kW kg-1 are observed for ASSCs device.

Figure 11 Comparative Ragone plot of β-MnO2/SS//O-SnS/SS ASSCs device with literature (inset photograph shows demonstration of ASSCs with 4 red, 4 yellow and 1 yellow and 1 green LEDs). Table 2. Comparison of the electrochemical performances of β-MnO2/SS//O-SnS/SS ASSCs device in this work with those reported in the literature.

Sr. No

Supercapacitor device

Electrolyte

Specific capacitance( Cs)

Energy density -1

(Whkg )

Power density (kWkg-1)

Electrochemica l stability

Ref.

(%) (Cycles)


28

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1

CuO@MnO2//MEGO

Na2SO4

49.2 F g-1at 22.1 0.25 A g-1

85.6

101.5 10000

2

rGO/MnO2/CB//rGO/C B

Na2SO4

209

21

89 after 1000

[47]

3

Sidiatom@MnO2//AG O

PVDFNa2SO4

65.1 F g-1 at 23.2 0.5 A g-1

0.105

84.8 after 2000

[48]

4

MnO2//AC

Na2SO4

15.84

0.885

88 after 1000

[49]

5

PANI//Carbon Maxsorb KNO3

-

11.46

45.6

-

[50]

6

PPy//Carbon Maxsorb

KNO3

-

7.64

48.3

-

[50]

7

PEDOT//Carbon Maxsorb

KNO3

-

3.82

54.1

-

[50]

8

MnO2//PANI

KNO3

-

5.86

42.1

-

[50]

9

MnO2//PPy

KNO3

-

7.37

62.8

-

[50]

10

MnO2//PEDOT

KNO3

-

13.5

120

-

[50]

11

MnO2//Fe3O4

K2SO4

21.5

8.1

10.2

-

[51]

12

MnO2//Carbon

K2SO4

31

17.3

19

-

[51]

13

MnO2//AC

Na2SO4

269

17.1

0.1

94 after 2000

[52]

14

MnO2//AC

K2SO4

21

10

16

77

[53]

15

GNCC//AC

KOH

288

19.5

5.6

102 after 10000

[54]

16

CNTs@NCS@MnO2//A C

Na2SO4

312.5

27.3

220

92.7 after 4000

[55]

17

GO/PPy//AC

LiCl/PVA

189.5 mF cm−2 at 10 mA cm−2

20

0.4

86 after 2000

[56]

18

MnO2//GO

Na2SO4

29.8

6.9

0.7

93 after 5000

[57]

19

CNT/MnO2/GR

Na2SO4/PV P gel

-

24.8

1.2

-

[58]

ZnO@MnO2//RGO

LiCl/PVA gel

0.23 0.52 F cm-3 at mWh 10mV s-1 cm-3

98.5 after 5000

[59]

20

24.3

after

[46]

0.133 Wcm-3


29


21

22

0.58 1.77 F cm-3 at mWh 1 mA cm-2 cm-3

CoSe2//MnO2

LiCl/PVA gel

SWCNTs//RuO2

H3PO4/PVA gel

23

Carbon aerogel//Co3O4

KOH-PVA gel

24

Graphene(ILCMG)//Ru O2–IL-CMG

H2SO4/PVA gel

25

Carbon cloth@T1 M Na2SO4 Nb2O5@MnO2//GO

26

β-MnO2/SS//O-SnS/SS

PVALiClO4

Page 30 of 45

0.282 Wcm-3

138 F g-1 at -1

94.8 after 2000

[60]

[61]

18.8

96

60~70 1000

17.9

0.750

85 after 1000

[62]

19.7

6.8

̶

[63]

2.25

85.7 after 2000

[66]

1.25

95.3 after 5000

This Work

8Ag

after

57.4 F g-1 1 Ag-1 175 F g-1 at 0.5 A g-1

319 F g-1at 31.76 0.2 A g-1 122 at 5 mV s

-1

29.8

The demonstration of fabricated β-MnO2/SS//O-SnS/SS ASSCs device is carried out with yellow, red, green light emitting diodes (LEDs)) with series connection of two ASSCs devices. One ASSCs device has +1.6 V operating potential windows, so, the series combination of ASSCs device gives 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 ASSCs device is discharged through 4 yellow, 4 red and one yellow and one green LEDs which glow with high intensity up to 2, 3 and 4 minutes, respectively. Inset of Figure 11 shows initial photographs captured during discharging of ASSCs device after 30 s charging. The discharging period of ASSCs device indicates excellent storing ability of β-MnO2/SS//O-SnS/SS ASSCs device with better ED as well as PD. 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


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1203 Fg-1 at 5 mV s-1, respectively. Asymmetric all-solid-state supercapacitors (β-MnO2/SS//OSnS/SS) fabricated from two optimum electrodes reach Cs of 122 Fg-1 (at 5 mV s-1), shows performance much higher than previous asymmetric supercapacitor based on MnO2 positive electrodes. The ASSCs device has excellent cycling performance (stable at 95.3% after only 5000 GCD cycles, 10 mA) denotes excellent ability of supercapacitor device. The large capacitance and high single ASSCs device voltage of +1.6 V allow high ED and PD of 29.8 Wh kg−1, 1.3 kW kg−1, respectively. Additionally, a series combination of two ASSCs device (+3.2 V) glows 4 yellow, 4 red and one yellow and one green LEDs up to 2, 3 and 4 min, confirming storage ability of SCs device. In conclusion, ASSCs device fabricated using the first combination of novel materials (β-MnO2 and O-SnS) is promising for future high energy density storage SCs. EXPERIMENTAL SECTION MATERIALS All chemicals were of analytical grade and used as it 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 current collector material. All solutions were prepared in double distilled water (DWW). 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, the KMnO4 was used as the precursor, while the methanol was used as the reducing agent. The matrix solutions were prepared by dissolving 0.1 M KMnO4 in 100 ml DDW. The prepared solution was stirred for 10 min at room temperature to make


31


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homogenous distribution of the KMnO4 reagent. Furthermore, 4 ml of methanol was added in above solution and whole solution added in a beaker containing vertically immersed SS substrate. This bath kept at room temperature for 12 h. After that, the blackish brown colored βMnO2/SS electrode was taken out from the bath, washed in DDW and dried at room temperature. The MnO2 content on SS substrate was calculated by weighting 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 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 min. Afterward, SnCl2 dissolved in DDW, 1 ml of TEA was mixed drop wise in each beaker solution, then 0.15 M of Na2S2O3 was added in the above solution. Likewise, dilute 1M HCl was added drop wise in solution till 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 time of 120, 240 and 360 min, to alter the surface morphology of thin film while 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 till the solution became clear and then


32

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slow stir for 6 h at room temperature in order to eliminate the bubbles engendered during the dissolution. Besides, to maintain q+ = q-, the weight ratio (1:1) between the β-MnO2/SS and OSnS/SS was adjusted using formula as follows24: M1 C1 × V1 = 10 M2 C2 × V2 Where, M1 and M2, C1 and C2 and V1 and V2 are the masses, Cs and potential windows of βMnO2 positive electrode and O-SnS negative electrode (TS2), respectively. The size of ASSCs device was about 5 cm × 5 cm. After packing of ASSCs device, it’s 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. CHARACTERIZATION OF β-MnO2 AND O-SnS THIN FILMS: The structural study of thin films was carried out by the XRD technique using Bruker AXS D8 advance model with copper radiation (Kα, λ = 1.54 Å). The chemical bonds existing in film materials were precisely inspected using Fourier transform infrared (FTIR) spectroscopy. The 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. Rame-Hart instrument was used for contact angle measurements. The Xray photoelectron spectroscopy (XPS) analysis was carried out for finding chemical composition and oxidation states of film materials using ESCALAB 250-Xi X-ray photoelectron spectrometer microprobe. The SSA and porosity were studied by the Brunauer-Emmett- Teller (BET) analysis and the pore size distribution was intended from Barrette-Joynere-Halenda (BJH) method. The electrochemical supercapacitive measurements were carried out by automatic battery cycler


33


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WBCS-3000 unit in a three electrode system of β-MnO2, O-SnS thin film on a SS substrate as a working electrode, saturated calomel electrode (SCE) as a reference electrode and platinum as a counter electrode. The area of β-MnO2 and O-SnS each electrode dipped into electrolyte were 1 cm2. The electrochemical impedance study was carried out by electrochemical workstation ZIVE MP1 unit within 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 β-MnO2/SS//OSnS/SS ASSCs device using two electrode system. ASSOCIATED CONTENT Supporting Information 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 (Histogram 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. AUTHOR INFORMATION Corresponding Author *Prof. C. D. Lokhande E-mail:- [email protected] ACKNOWLEDGEMENT The authors are grateful to Department of Science and Technology-Science and Engineering Research Board (DST-SERB), New Delhi, India for their financial support through


34

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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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Table of content (TOC)

Novel chemical bath synthesis of positive β-MnO2 and negative O-SnS electrodes as a new promising electrode preparation strategy to enhance storage capacity of asymmetric supercapacitor with increment in the potential window of +1.6 V for a single device, CV curves are in the range of -0.4 to -1.2 V/SCE and 0 to +0.8 V/SCE for negative and positive electrodes gives +1.6 V to SS/β-MnO2//O-SnS/SS ASSCs device. Achieved energy and power density values included in Ragone plot besides it compared with previous ASSCs.


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High Performance All-Solid-State Asymmetric Supercapacitor Device

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