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Hexagonal VS2 anchored MWCNTs: First approach to design flexible solid-state symmetric supercapacitor device Bidhan Pandit, Swapnil S Karade, and Babasaheb R Sankapal ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b13908 • Publication Date (Web): 04 Dec 2017 Downloaded from http://pubs.acs.org on December 4, 2017

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ACS Applied Materials & Interfaces

Hexagonal VS2 anchored MWCNTs: First approach to design flexible solidstate symmetric supercapacitor device Bidhan Pandit, Swapnil S. Karade, Babasaheb R. Sankapal* Nano Materials and Device Laboratory, Department of Physics, Visvesvaraya National Institute of Technology, South Ambazari Road, Nagpur-440010, Maharashtra, India.

Abstract Transition metal chalcogenides (TMCs) embedded with carbon network is gaining much courtesy due to high power capability which can be easily integrated to portable electronic devices. Facile chemical route has been explored to synthesize hexagonal structured VS2 nanoparticles onto multi-walled carbon nanotubes (MWCNTs) matrix. Such surface modified VS2/MWCNTs electrode has boosted the electrochemical performance to reach high capacitance to 830 F/g and excellent stability to 95.9 % over 10000 cycles. Designed flexible solid-state symmetric supercapacitor device (FSSD) with a wide voltage window of 1.6 V exhibited maximum gain in specific capacitance value of 182 F/g at scan rate of 2 mV/s along with specific energy of 42 Wh/kg and a superb stability of 93.2 % over 5000 cycles. As a practical approach, FSSD has lightened up ‘VNIT’ panel consisting of 21 red LEDs. Keywords: SILAR, VS2/MWCNTs, supercapacitor, flexible solid-state supercapacitor device

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Introduction Incorporation of conducting transition metal oxides, chalcogenides and conducting polymers with nanostructured carbon-based materials have gained promising approach to boost the electrochemical behavior. Mechanism behind the charge storage for carbon based materials is principally centered on charge accumulation by developing electric double layer at electrodeelectrolyte interface. Amalgamation of highly conducting multi-walled carbon nanotubes (MWCNTs) with the specific crystallographic orientated nanostructures can provide a wellordered system exhibiting superb electrochemical activity with excellent stability. MWCNT modified surface architecture through the synergistic effect improves the number of electrochemical active sites.1-2 Furthermore, it enhances the electronic behavior of active electrode material which further boosts the reaction kinetics with electrolyte ions. Thus, hybrid approach overcomes weaknesses of individual components during electrochemical activities; resulting in superior performances which is limited by individuals. Nowadays, transition metal chalcogenides (TMCs) have attracted snowballing interest to develop superior electrode material for supercapacitor application due to their unique physical and chemical properties.3 Nie et al. have reported the enhancement in the capacitance value of Bi2S3 nanorods to 396 F/g by forming composites with reduced graphene oxide (rGO).4 Zhu el al. have obtained the highest specific capacitance of 927-583 F/g with respect to different specific currents from 4.08-10.2 A/g for NiS hollow spheres synthesized by a template-engaged conversion route.5 A simple chemical route has been adopted by Pande et al. to synthesize HgS nanoparticles and achieved specific capacitance of 446 F/g.6 Ratha el al. have synthesized WS2/rGO hybrid material using hydrothermal method to enhance the specific capacitance value as high as 350 F/g.7 Karade et al. have adopted simple chemical route to synthesize MoS2 2 ACS Paragon Plus Environment

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nanoflakes which exhibited maximum specific capacitance of 576 F/g.8 Moreover, Wang et al. have enhanced the specific capacitance to 314 F/g for CoS2 by forming CoS2-graphene nanocomposite.9 Among TMCs family, VS2 has a unique S-V-S layered structure stacked together by weak Van der Waals force involving an interlayer spacing of 5.76 Å.10-11 This framework offers a large number of open channels for electrolyte ions with enhanced intercalation/deintercalation and the metallic property to boost the charge transfer process.12 Unique structure and excellent electrical conductivity of VS2 including its low-cost and source abundance, VS2 explores its candidature towards development of promising electrode material for supercapacitor application. Vanadium based chalcogenides are nearly excluded members from research field meanwhile from its discovery. Recently, a few reports have been emerged related to synthesis and application of vanadium chalcogenides and their composites. Lui et al. have synthesized template-free VS4 nanostructured material through hydrothermal technique for photocatalysis application.13 Fang et al. have used one-pot synthesis process to prepare VS2/graphene nanocomposites (VS2/GNS) as lithium ion battery cathodes.14 Xu et al. prepared VS4/graphene nanocomposites through hydrothermal method with high rate capability anode material for lithium ion batteries.15 VS2 nanoflower and gold nanoparticles glassy carbon electrode was synthesized by Huang et al. for electrochemical biosensor.16 For sodium ion storage application, Liao et al. prepared Na2Ti2O5 nanowire arrays coated with VS2 nanosheets (NTO–VS2) with enhanced capacity and rate capability.17 Present investigation highlights the synthesis of VS2 hexagons on the hexagonal matrix of MWCNTs nano-network by using simple and cost-effective successive ionic layer adsorption and reaction (SILAR) method as the first report. Due to the structural and conductive support, 3 ACS Paragon Plus Environment


‘dip and dry’ processed MWCNTs served as core for VS2 nanostructure as shell (figure 1a). Such surface modified highly conductive network may facilitate excellent faradic charge transfer. Due to the prohibited charge accumulation, degradation of structural conformation of VS2 has been retarded during charge-discharge process. Thus, rapid ion diffusion and fast charge-discharge reaction kinetics have been improved at the conductive interface between VS2 and MWCNTs by decreasing overall resistance. Flexible solid-state symmetric supercapacitor device (FSSD) has been designed based on VS2/MWCNTs electrodes with the aid of low-cost PVA-LiClO4 gel as a mediator. The developed nanostructure properties are well supported through in-depth characterizations. The exhibition of grander supercapacitive characteristics including long-term chemical and mechanical stabilities can reconnoiter the engineered device for high-tech flexible applications as energy storage devices.

Results and discussions Structural study The structure of SILAR deposited VS2/ MWCNTs on stainless steel (SS) substrate was confirmed by X-ray diffraction (XRD) studies with reference to SS, MWCNTs/SS and VS2/SS (figure 1b). Two distinct peaks at 15.4 and 35.7º are recognized in both VS2 and VS2/MWCNTs correspond to the (001) and (011) planes of hexagonal VS2. Moreover, VS2/MWCNTs film undoubtedly specifies an extra peak at 25.82º (signifies as ‘Ψ’) which resembles to graphitic (002) plane of MWCNTs.18 The observed diffraction angles and interplanar spacing are in well agreement with standard JCPDS card no. 89-1640. The emergence of strong peaks labeled as ‘∆’ is assigned to use of SS substrate as reference.


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The compositional bond structure and phase analysis of synthesized materials have been explored by Fourier transform infrared spectroscopy (FTIR) by comparing MWCNTs, VS2, and VS2/MWCNTs spectra and are depicted in figure 1c. The sharp absorption band at 525 and 681 cm-1 are clearly assigned to the doubly bounded and doubly bridged S2− from V–S–V whereas 982 cm-1 is due to stretching of terminal S (V=S).14, 19 Additional peaks appeared at 1633 and 3440 cm-1 are attributed towards bending vibration of H–O–H and stretching vibrations of O–H. The peak at 2365 cm-1 is mainly assigned to stretching modes associated to the carbon backbone.20 This confirms the deposition of VS2 on MWCNTs with solid collaboration among both components. In order to characterize the nanostructure of composite electrode, Raman spectroscopy of MWCNTs, VS2, and VS2/MWCNTs was performed and shown in figure 1d. Four characteristic peaks observed at 268, 303, 396 and 407 cm-1 is associated with VS2 and VS2/MWCNTs samples. The most intense peak at 268 cm-1 is associated with the E1g vibration mode of VS2 with the hexagonal curvature. The peaks originated at 396 and 407 cm-1 are attributed to the in plane (E ) displacements and the out-of-plane (A ) symmetric displacements of sulfur atoms of

VS2.14, 21 Two prominent peaks are also shown for MWCNTs and VS2/MWCNTs sample at 1342 and 1574 cm-1 related to sp3 (D band) and sp2 (G band) bonded carbon atoms, respectively.22 The peak intensity of both band for VS2/MWCNTs has enhanced as compared to bare MWCNTs due to local electromagnetic field affected surface-enhanced Raman scattering (SERS) for the existence of VS2 nanostructure.23 The relative intensity ratio shows the information about graphitic and disordered carbons. The ID/IG ratio as 0.84 for composite corresponds to the enhanced electronic conductivity for electrode.



The oxidation states of V and S were supported by X-ray photoelectron spectroscopy (XPS) analysis (figure 2a-c). Figure 2a shows complete survey spectrum of VS2/MWCNTs. The peaks at 516.6 and 523.7 eV are attributed to V 2p3/2 and V 2p1/2 levels, indicating V4+ oxidation state (figure 2b).21, 24 An asymmetric behavior has been observed for V 2p3/2 due to the inclusion of a very week peak around 515.4 eV. The additional component in V 2p3/2 signal can be assigned to the presence of V3+ oxidation state.25 Hence, very minute inclusion of V2S3 cannot be excluded entirely. The divalent sulfide ions was confirmed by S 2p with two peaks at 162.8 and 163.9 eV which can be correlated to S 2p3/2 and S 2p1/2, respectively (figure 2c). The peak at 284 eV is attributed to C 1s peak, confirming the presence of MWCNTs.26 The results strongly support the existence of VS2 over MWCNTs. Thermal stability of the composite material has been tested by using thermogravimetric analysis (TGA) and derivative thermal gravimetric (DTG) plots which is depicted in figure 2d. A continuous weight loss has been observed below 200 ºC due to desorption of water molecules on and within the surface layers.27 The second weight loss of 7 % at around 328 ºC can be attributed to the removal of sulfur from VS2. The sudden weight loss between 520-650 ºC refers to the complete decomposition of carbon.28 At the end of the curve, weight continues to decrease slowly due to oxidation of V4+ to relatively greater and stable V5+ oxidation state with a precise phase transition.19 Morphological analysis Proper composition and morphological structure are the key components to realize the full potential of hybrid electrode material compared to conventional electrode material. As displayed by field emission scanning electron microscopy (FESEM) images (figure 3a), MWCNTs consist of mesoporous structure due to the most complex web-like nano-network 6 ACS Paragon Plus Environment

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which was favorable to anchor VS2 by using SILAR method as the precursor finds easy path to penetrate from top to bottom portion of MWCNTs film. Figure 3b depicts uniformly distributed VS2 nanoparticles over the SS substrate as reference. After grafting VS2 nanoparticles on MWCNTs, the composite VS2/MWCNTs (figure 3c) can provide relatively porous and active nanostructure which may contribute to the rapid electrochemical reactions during chargedischarge process. At higher magnification, FESEM image reveals that the nanoparticles are encapsulated to the outer surface of MWCNTs with well surface coverage (figure 3d). This composite facilitates worthy interface which can support well-organized charge transport along the radial direction of MWCNTs over outer VS2 layer. The chemical compositions displayed the appearance of strong peaks in energy dispersive spectroscopy (EDS) spectra for individual components from VS2 and VS2/MWCNTs thin films, respectively (Supplementary information, S1). EDS results reveal that the atomic ratio of V:S in the VS2/MWCNTs thin film is quite away from the stoichiometry. The discrepancy can be assigned to the local inhomogeneity of sample, as the small area of sample cannot be an objective representation of whole sample and hence, cannot be used as quantitative analysis. Similar type of chalcogenide deficiency was reported by Wang et al. for three TMD (Transition Metal Dichalcogenides)graphene composites.29 Hu et al. described the variety of the component and ratio of Ni and Co with sulfide in EDS spectra for Ni-Co sulfide and attributed to the pseudo Kirkendall effect.30 Similar research articles are emerged showing the discrepancy in EDS analysis from the stoichiometric ratio.31-32 Additionally, EDS elemental mapping analysis based on FESEM has been performed to get a sure and clear insight (Supplementary information, S2). The high resolution transmission electron microscope (HRTEM) images shown in figure 4a-c reveals irregular shaped hexagonal VS2 nanoparticles embedded on MWCNTs which are 7 ACS Paragon Plus Environment


not visualized in FESEM images. Thermodynamically, trigonal and hexagonal are the two stable structures for VS2 to exist.33 Due to monolayer formation by used SILAR method which favors hexagonal form of VS2 at initial growth stage along with use of hexagonal crystalline network of MWCNTs as template, the growth of VS2 was restricted to hexagonal surface architecture. Hence, obtained hexagonal architecture is the complex contribution of used SILAR method along with the use of MWCNT network. The inset of Figure 4b shows the selected area electron diffraction (SAED) pattern recorded for the hybrid VS2/MWCNTs electrode that exhibited (012) plane confirming VS2 structure. Figure 4c shows strong synergic encapsulation between MWCNTs and VS2. Using in-depth analysis, two distinct planes with d-spacing of 0.37 nm and 0.23 nm have been observed in the hexagons where one appeared at the boundary and other in core of the particle respectively and are depicted in figure 4d. The well resolved lattice fringes with inter planar spacing of 0.23 nm has been visualized in the core of the particle corresponding to the (011) plane of hexagonal VS2 (figure 4d) are well supported by the intense peak observed in XRD (JCPDS card no. 89-1640) studies. The appearance of planes with 0.37 nm d-spacing can be assigned to minute content of V2S3 (JCPDS card no. 37-1115) as referred by XPS analysis. Furthermore, similar outer surface with crystalline pattern have been observed by Wirth et al. during the growth of CNTs using nickel catalyst.34 The reshaping of nanoparticles initiated for equilibrium shape and the sequence recurs.35 The smooth and crystalline surface planes of particle are due to surface transformation towards hexagonal growth.36-37 HRTEM elemental mapping was performed to locate the distribution profile of associated components on lattice structure for as-synthesized VS2/MWCNTs sample. Figure 5a-d shows the distribution of V and S elements on the MWCNTs surface which confirms the formation of VS2 on MWCNTs whereas appearance of low content of carbon might be due to use of carbon 8 ACS Paragon Plus Environment

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grid for EDS mapping which would weaken the relative intensity of carbon element present in MWCNT.38 Supercapacitive performance of electrode The electrochemical activities of MWCNTs, VS2 and VS2/MWCNTs were performed by using cyclic voltammetry (CV) in optimized 2 M KCl (Supplementary information, S3) in the potential frame of -0.6 to 0.2 V at scan rate of 100 mV/s (figure 6a). The composite electrode clearly exhibits a prompt current response in both positive and negative sweeps compared to bare MWCNTs and VS2. The CV plots of MWCNTs manifest the rectangular shape which corresponds to EDLC behavior. Though there are no distinct redox peaks in both the CV curves of VS2 and VS2/MWCNTs rather they deviate from EDLC derived ideal rectangular shape, implying the occurrence of reversible redox reactions. The CV curves at different scan rates have been analyzed and depicted in figure 6b. The origin of supercapacitive activities for VS2/MWCNTs composite is related to the dual charge-storage approach39 such as surface adsorption and intercalation/deintercalation of K+ ions as (VS ) + xK + xe ⇌ (V − ) (1) and

the

intercalation

of

K+

ions

related

to

the

following

reversible

reaction:

xK + xe + VS ⇌ K V (2) Mainly, the electrochemical activities can be monitored through the surface and diffusion controlled charge contributions which altogether utilize the electrochemical active surface area. The correlation of outer charge (qo) narrates the outer region of electrode exposed to the electrolyte, whereas the inner charge (qi) mainly defines the inner part of electrode such as grain boundaries, pores, and voids.40 At relatively higher scan rates, limitation to the diffusion process 9 ACS Paragon Plus Environment


has been occurred to more inner accessible sites i.e. outer surface of the electrode. So, ‘qo’ can be calculated from ‘q’ vs ‘v−1/2’ (figure 6c) assuming ‘v’ tends to infinite. Contrary, the overall charge (qt) can be evaluated after the extrapolation obtained from ‘1/q’ vs ‘v1/2’ (figure 6d) assuming ‘v’ tends to zero.18,

41

The analysis gives 72 % dominance of surface controlled

contribution over the diffusion related contribution as conclusion (Supplementary information, S4). All the electrochemical parameters were evaluated by using the standard equations (Supplementary information, S5). The composite electrode reveals a maximum specific capacitance of 830 F/g at scan rate of 2 mV/s as shown in figure 6e. The plot also demonstrates the decrease in the specific capacitance with increase in the scan rate due to limited electrochemical process. Moreover, the CV shapes at higher scan rates are almost identical, suggesting high rate capability of as-synthesized electrode. As stability of active electrode material is concerned, cyclic repeatability of redox active material is not as decent as EDLCs. As we all know, degradation of active electrode material is the main issue for most of the reversible redox active electrode materials. To check the chemical and mechanical stability of electrode material, the CV loops were repeated for 10000 cycles at scan rate of 100 mV/s and are depicted in figure 6f. The electrode shows 95.9 % cyclic retention even after 10000 cycles, exhibiting a long-term cycling stability which can be attributed to the integrated MWCNTs matrix anchored with its redox active VS2 nanoparticles.42 The unperturbed and factual shape of CV curves even after such massive number of cycles confirms higher rate capability of VS2 (inset, figure 6f). The FESEM image of VS2/MWCNTs electrode also reveals minimum degradation of material by sustaining the initial structure even after large cycles (Supplementary information, S6).


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In order to compare the charge storage actions of VS2, MWCNTs and VS2/MWCNTs, the galvanostatic charge-discharge (GCD) curves have been analyzed with constant current density of 1.5 mA/cm2 and are depicted in figure 7a. Importantly, the charge and discharge regions are not perfectly linear which clearly demonstrates the reversible redox mechanism related to intercalation/deintercalation of electrolyte ions onto VS2. The GCD variation of VS2/MWCNTs was analyzed at different current densities and is depicted in figure 7b. At the initial stage of discharge curve, sudden drop in potential has been perceived; termed as ohmic iR drop where i and R represent the current and resistance originating from the contributions of small resistances appeared at interface of SS to MWCNTs and MWCNTs to VS2 nanoparticles.43 Remarkably, the composite electrode exhibits very high specific capacitance of 706 F/g at constant current density of 1.5 mA/cm2 (inset, figure 7b). The exceptionally superior specific capacitance is attributed to the modified surface architecture which facilitates the surface adsorption and reversible redox reaction of electrolyte cations (K+). Design and enactment of flexible solid-state symmetric supercapacitor device (FSSD) To fabricate flexible solid-state device, highly conductive gel is the requisite and hence, PVA-LiClO4 gel was selected (Supplementary information, S7). This gel was prepared by using 6 g of PVA in 30 ml of double distilled water (DDW) at 343 K under vigorous stirring to get viscous appearance. Next, 30 ml solution of LiClO4 (5 g) was added to the previously prepared solution under the same condition to have a viscous glue. Formed clear and transparent gel was used to fabricate solid-state symmetric device. A thin layer of the prepared gel was encrusted on electrode and dried at 300 K to remove hydroxide content. Two symmetric electrodes were sandwiched together by embossing 1 ton pressure normal to the surface for 12 h



to improve good adhesion and mechanical strength towards formation of flexible solid-state symmetric device (figure 7c). The fabricated FSSD has broaden voltage window of 0 - 1.6 V as shown in figure 8a. The CV curves for FSSD at different scan rates ranging from 2 to 100 mV/s have been depicted in figure 8b. The symmetric increment of specific current with respect to the escalation in scan rate along with unperturbed CV curves strappingly suggests the strong capacitive properties and high rate capability of fabricated FSSD. Electrolyte cations (Li+ ions) easily intercalate to the porous electro-active cavities of electrode material and exhibits outstanding electrochemical behavior44 as follows: xLi + xe + VS ⇌ Li V (3) Patil et al.45 have demonstrated symmetric NiS device using gel electrolyte and attained specific capacitance of 55.83 F/g whereas MnO2//MnO2 attained 110 F/g of specific capacitance as reported by Chodankar et al.46 Moreover, Gund et al. have formed MnO2//Fe2O3 asymmetric device to enhance the specific capacitance to 92 F/g.47 Existing VS2/MWCNTs shows maximum specific

capacitance of 182 F/g at scan rate of 2 mV/s. The variation of specific capacitance with scan rate has been depicted in figure 8c. To check cyclic stability, CV cycles were repeated continuously for 5000 times to get a retention status (figure 8d). Impressively, the device exhibited 93.2 % retention over 5000 cycles which is much better than the previously reported devices.45-47 For in-depth evaluation of FSSD, GCD has been studied at various current densities and presented in figure 8e. The electrode shows almost insignificant iR drops at different current densities implying low internal resistance of electrode. Contrary in device prospective, various 12 ACS Paragon Plus Environment

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unwanted resistive factors such as separator, package cell, and current controller-electrode connection are involved, resulting in the comparatively higher internal resistance as well as iR drop with respect to electrode configuration (Supplementary information, S8). The device exhibits specific capacitance of 118 F/g at current density of 2 mA/cm2 by revealing a small iR drop due to the good ionic conductivity of PVA-LiClO4 gel (inset, figure 8e). The superior results support that the device fabricated with PVA-LiClO4 gel is a strong contending design for high energy as well as power supercapacitor device. The structural improvement in hybrid electrode boosts the power of typical EDLCs or pseudocapacitors. Figure 8f shows the Ragone plot related from specific energy and power of the formed device. Dubal et al. have synthesized rGO-PMo12 and fabricated symmetric device having specific energy of 17.2 Wh/kg.48 In case of MoS2/Carbon cloth symmetric device reported by Javed et al., the device exhibited specific energy of 5.42 Wh/kg.49 In present case, the hybrid device exhibits a maximum specific energy of 42 Wh/kg

without sacrificing specific power of 2.8 kW/kg which is superior to previously reported solidstate supercapacitor devices (table 1). Most importantly, it delivers maximum specific power of 4.8 kW/kg with a downfall in specific energy to 24 Wh/kg. To get a detail capacitive and resistive internal activities of FSSD, electrochemical impedance spectroscopy (EIS) was performed and are depicted in figure 9a as Nyquist plot in frequency range of 100 kHz to 100 mHz with a 0 V bias potential. In high and low frequency regions, two vital characteristics were perceived which can be ascribed as various resistance phenomena occurred during different interfacial activities in reversible redox reactions. There is a discontinuity in charge transfer process at electrode-electrolyte interface due to conductive nature between electrode material (electronic conductivity) and solid electrolyte (ionic conductivity). The corresponding resistance involved in faradaic activities is charge transfer



resistance (RCT) that can be evaluated by the diameter of small arc at a high frequency region. The total solution resistance (RS) is referred by X-axis intercept in Nyquist plot.50 The evaluated values of RS (0.7 Ω/cm2) and RCT (11.5 Ω/cm2) are mainly attributed to satisfactory penetration of electrolyte ions to establish excellent electrochemical performance. The impedance characteristics were analyzed by the complex nonlinear least squares (CNLS) fitting method based on Randles equivalent circuit as depicted in inset of figure 9a. Mainly, constant phase element (CPE) arises due to the inclusion of inherent factors such as active disorder activity of associated diffusion and diverse relaxation time due to inhomogeneity at the electrode-electrolyte interface, porosity of electrode material and most importantly, nature of the used electrolyte. As intercalation/deintercalation is the cause of redox mechanism of hybrid electrode, the two CPE elements involved in circuit is directly related to porosity and semi-infinite intercalation of Li+ ions.51 Warburg component specifies the shift from high to low frequency region.52 The supercapacitive merit can be described as relaxation time constant (τ# ) which clearly defines the boundary between the resistive and capacitive behaviors. The low value of τ# corresponds to high power posture which is in favor of supercapacitive eccentricity. The value of

τ# usually can be calculated from τ# = using imaginary frequency component (C' ) vs.

%

frequency (f) plot53 where C' =

Z′ (4) 2πf ∣ Z ∣

Here, f# and Z specifies the characteristic frequency and impedance respectively.


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The estimated value of τ# is 176.68 ms (figure 9b) which clearly indicates the fast charge transport phenomena to the inner pores during redox reactions. The frequency dependent phase angle curve referred as Bode plot which is important to explain the quality of active electrode material. An ideal capacitive component indicates exact 90° phase angle.54 From the figure 9c, it is appreciated that the phase angle is close to -74.4° in low frequency region with a shallower slope due to the reversible redox behavior; suggesting maximum sustention of perfect capacitive behavior. Moreover, τ# often entitled supercapacitor

factor of merit which can also be estimated by the same equation τ# = with a anticipated phase

%

angle of −45° as in the present case, capacitive and resistive impedances are undistinguishable.55 The τ# value calculated from Bode plot is 112.23 ms which is comparable with early pronounced Nyquist plot consequence. Moreover, it is lower than that of the carbon-based macroscopic devices (>0.1 s) and activated carbon-based microdevices (700 ms).56 The flexible approach is advantageous in exploring portable and bendable electronic devices. A series of CV measurements were performed with respect to the bending angles ranging from 0 to 175° with a step of 30° at a fixed scan rate of 100 mV/s. The FSSD shows excellent flexibility and mechanical stability by exhibiting 97.4 % capacitive retention at a bending angle of 175° (figure 9d). The unperturbed CVs at each bend confirm high quality adhesion of sample on substrate surface along with favorable interface provided by PVA-LiClO4 gel electrolyte between two active electrodes. The potential profile with charge state based on VS2/MWCNTs symmetric device has been schematically illustrated in figure 9e. Both the positive and negative electrodes were assembled by using same electrode material in the symmetric form. The symmetric amount of 15 ACS Paragon Plus Environment


charges are stored in both the electrodes through an increase in potential difference (∆V)total. Two symmetric devices were connected in series and charged with 3.2 V which easily lighten up ‘VNIT panel’ consisting of 21 red LEDs for 60 s with remarkable intensity (figure 9f-h) which clearly signifies the practical applicability as working model (Supplementary video).

Conclusions A facile, time-efficient, eco-friendly, economical and industry scalable successive ionic layer adsorption and reaction (SILAR) strategy has been developed to synthesize VS2 thin film with unique hexagonal surface architecture anchored onto MWCNTs for supercapacitor application. Flexible solid-state supercapacitor device was designed by using VS2/MWCNTs electrodes with aid of PVA-LiClO4 gel electrolyte. The stable cyclic behavior and higher specific energy of formed FSSD clearly support the well matured comportment of formed supercapacitive device. Moreover, lightweight and flexible device offers a great potential towards flexible energy storage devices which can be easily integrated with commercial portable electronics.

Experimental Encapsulation of hexagonal VS2 over MWCNTs surface Thin film of MWCNTs were synthesized as per well-established literature.18 In brief, 95% pure MWCNTs (5–15 µm as length and 15–20 nm as outward diameter) were refluxed using H2O2 at 90 °C for 48 h in order to anchor oxygenated functional groups and to remove amorphous carbon derivatives. The obtained residue was rinsed repeatedly using double distilled water (DDW) for several times and dried at 60 °C for 12 h. To obtain stable dispersion,


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sonication of 0.125 g of MWCNTs was performed in Triton X-100 surfactant with 25 ml of DDW (Tx-100:DDW::1:100) for 1 h. Mirror cleaned stainless steel (SS) substrate was dipped vertically in the solution for 20 s to adsorb MWCNTs onto SS substrate and dehydrated through IR source for solvent evaporation. Repetition of process for 20 times retains optimum coating of MWCNTs on SS and to form appropriate nucleation sites with high surface area with interconnected nano-network for deposition of VS2. Nanostructured thin film of VS2 on SS substrate was deposited through SILAR using 0.1 M vanadyl sulfate (VOSO4) as cationic precursor whereas 0.2 M sodium sulfide (Na2S) as anionic precursor. First mirror polished SS substrate was dipped into cationic solution for 60 s to adsorb cations. In the next step, reaction was accomplished by immersing the same substrate in to anionic solution for 60 s. The cycles were repeated for 80 times for optimum coating of VS2 over MWCNTs (Supplementary information, S9). To remove extra growth, intermediate rinsing step was involved for 20 s in DDW after each immersion. Hexagonal VS2 nanostructure on MWCNTs was deposited through SILAR. Vanadyl sulfate provides stable vanadyl (VO2+) ion which further helps to form VS2 film. VOSO1 = VO + SO 1 (5) During first immersion of MWCNTs/SS substrate to the VOSO4 solution, VO2+ ions get adsorbed onto the substrate. On the other hand, when VO2+ adsorbed MWCNTs/SS substrate is immersed into anionic precursor; S2- present in Na2S reacts with VO2+ to provide blackish-brown VS2 film as deposition. VO + 2S + H O → VS ↓ +2OH (6)



Characterizations The structure, chemical bonding, and morphology were characterized by XRD (Bruker AXS D8 Advance having Cuk 8 source), FTIR (Thermo Nicolet, Avatar 370), Raman spectroscopy (Renishaw INVIA, laser wavelength 532 nm), XPS (VG Multilab 2000; Thermo VG Scientific, UK), TGA (TG-DTA 7200, Hitachi), FESEM (JEOL Model JSM - 6390LV) coupled with EDS, and HRTEM (JEOL 2100 with LaB6 source) including SAED pattern. The electrochemical investigations such as CV, GCD, and EIS were analyzed by using potentiostat/galvanostat using PARSTAT-4000, (Princeton Applied Research, USA).

Associated content Supplementary information EDS analysis of VS2 and VS2/MWCNTs thin films, EDS elemental mapping of VS2/MWCNTs thin film, Electrolyte variation of VS2/MWCNTs electrode, Inner and outer charge contributions of VS2/MWCNTs electrode, Electrochemical characterizations, Surface morphology after cyclic stability studies, Comparative electrochemical behaviors of FSSD in PVA-KCl and PVA-LiClO4 gel electrolytes, iR drop and Coulombic efficiency of FSSD, SILAR cycle variation Supplementary video Practical demonstration of FSSD discharging through ‘VNIT’ panel consisting 21 red LEDs

Author information *Corresponding Author: [email protected]; [email protected] Contact No.: +91 (712) 2801170; Fax No.: +91 (712) 2223230 ORCID Bidhan Pandit - 0000-0003-4656-9289 18 ACS Paragon Plus Environment

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Swapnil S. Karade - 0000-0002-7503-0912 Babasaheb R. Sankapal - 0000-0002-7464-9633 Author Contributions B.P. performed the experiments, electrochemical characterizations, data analysis and wrote the manuscript. B.P., S.S.K., and B.R.S. reviewed the data and manuscript. To the preparation of manuscript, all authors contributed equally. Notes The authors declare no competing financial interest.

Acknowledgements BP and SSK gratefully acknowledge VNIT, Nagpur and BRS acknowledges to DST/TMD/MES/2k16/09 project, Govt. of India.



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Table captions Table 1: Summary of the performance of FSSD with several reported solid-state supercapacitor devices. Figure captions Figure 1 (a) Schematic illustrating the deposition of hexagonal VS2 on MWCNTs, (b) XRD patterns of bare stainless steel (SS), MWCNTs, VS2, and VS2/MWCNTs thin films, (c) FTIR and (d) Raman spectra of MWCNTs, VS2 and VS2/MWCNTs samples. Figure 2 (a) XPS survey spectrum of VS2/MWCNTs, (b, c) core level XPS spectra of VS2 for V 2p and S 2p, (d) TGA graph of VS2/MWCNTs composite. Figure 3 (a-c) FESEM images of as-prepared MWCNTs, VS2, and VS2/MWCNTs thin film on SS substrate at 1 μm magnification, (d) FESEM image of VS2/MWCNTs at magnification 200 nm. Figure 4 (a-d) HRTEM images of VS2/MWCNTs at different magnifications, inset of figure (b) shows SAED pattern of VS2/MWCNTs. Figure 5 (a-d) EDS elemental mapping analysis; the red, green and blue color represents carbon (C), vanadium (V), and sulfur (S) respectively. Figure 6 Electrochemical performances in 2 M KCl electrolyte. (a) CV curves for MWCNTs, VS2 and VS2/MWCNTs samples at scan rate of 100 mV/s, (b) CV curves of VS2/MWCNTs at different scan rates ranging from 2 to 100 mV/s, (c) q vs. v−1/2, and (d) q-1 vs. v1/2 plots derived from CVs at different scan rates, confirming the supercapacitive characteristics, (e) specific capacitance at a function of scan rate, (f) cycling stability for 10000 cycles, inset shows the CV curves for different cycle numbers at 100 mV/s scan rate.



Figure 7 (a) GCD curves of MWCNTs, VS2 and VS2/MWCNTs at specific current of 1.5 mA/cm2, (b) GCD curves at different current densities ranging from 1.5 to 3 mA/cm2, inset shows specific capacitance at a function of current density, (c) Schematic representation of the fabricated FSSD device based on VS2/MWCNTs electrode with PVA-LiClO4 gel electrolyte. Figure 8 Electrochemical performance of FSSD device with PVA-LiClO4 gel electrolyte. (a) CV curves for different voltage windows ranging from 1 to 1.6 V at scan rate 100 mV/s, (b) CV curves at different scan rates ranging from 2 to 100 mV/s with voltage window 1.6 V, (c) specific capacitance at a function of scan rate, (d) cycling stability over 5000 cycles at 100 mV/s scan rate, inset shows CV curves for different cycle numbers at scan rate of 100 mV/s, (e) GCD curves at different current densities ranging from 2 to 3.5 mA/cm2, inset shows specific capacitance as a function of current density, (f) Ragone plot compared with previously reported values. Figure 9 (a) Nyquist plot of impedance from 100 mHz to 100 kHz, inset shows corresponding equivalent circuit, (b) real and imaginary capacitance vs. frequency plot, (c) Bode plot, (d) capacitance retention at different bending angles, inset shows CV curves with different bending angles at scan rate of 100 mV/s, (e) Schematic representation of charge-voltage profile, (f-h) actual demonstration of FSSD discharging through ‘VNIT’ panel consisting 21 red LEDs in parallel arrangement for 0 s, 30 s, and 60 s respectively.


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Table 1

Electrode

Cell

materials

remarks

Cyclic stability

Specific

Specific

Specific

capacitance

energy

power

Retention

(F/g)

(Wh/kg)

(kW/kg)

(%)

Cycles

Ref.

NiS

Symmetric

55.83

9.3

0.67

90

1500

45

MnO2

Symmetric

110

23

1.9

92

2200

46

MnO2//Fe2O3

Asymmetric

92

41.8

1.3

91

3000

47

rGO-PMo12

Symmetric

-

17.20

0.13

89-95

5000

48

Symmetric

368

5.42

0.13

96.5

5000

49

Symmetric

182

42

2.8

93.2

5000

MoS2/Carbon cloth VS2/MWCNTs

Present work



Figure 1


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Figure 2



Figure 3


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Figure 4



Figure 5


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Figure 6



Figure 7


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Figure 8



Figure 9


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Graphical abstract

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Hexagonal VS2 Anchored MWCNTs: First ... - ACS Publications

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