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A Mesoporous High Performance Supercapacitor Electrode Based on Polypyrrole Wrapped Iron Oxide Decorated Nanostructured Cobalt Vanadium Oxide Hydrate with Enhanced Electrochemical Capacitance Anirban Maitra, Amit Kumar Das, Sumanta Kumar Karan, Sarbaranjan Paria, Ranadip Bera, and Bhanu Bhusan Khatua Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.6b04449 • Publication Date (Web): 09 Feb 2017 Downloaded from http://pubs.acs.org on February 16, 2017

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Industrial & Engineering Chemistry Research

A Mesoporous High Performance Supercapacitor Electrode Based on Polypyrrole Wrapped Iron Oxide Decorated Nanostructured Cobalt Vanadium Oxide Hydrate with Enhanced Electrochemical Capacitance Anirban Maitra, Amit Kumar Das, Sumanta Kumar Karan, Sarbaranjan Paria, Ranadip Bera, Bhanu Bhusan Khatua* Materials Science Centre, Indian Institute of Technology Kharagpur, Kharagpur-721302, West Bengal, India.

*Corresponding Author Dr. B. B. Khatua (Email: [email protected]). Materials Science Centre, Indian Institute of Technology, Kharagpur –721302, India. Tel.:+91-3222-283982 ACS Paragon Plus Environment

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ABSTRACT: Here we demonstrate synthesis of grass-like cobalt vanadium oxide hydrate (CVO) nanocanes arrays followed by decoration of CVO by iron oxide nanospheres (FeO@CVO) using iron nitrate and CVO through a cost-effective hydrothermal method. Finally, a high performance robust mesoporous hybrid composite electrode (PPy/FeO@CVO) was fabricated through wrapping up of polypyrrole (PPy) over FeO@CVO using low temperature in-situ oxidative polymerization of pyrrole. Electrochemical studies of PPy/FeO@CVO with 1 M KOH reveals highest specific capacitance of 1202 F/g with exceptionally high cyclic stability at 1 A/g, in three-electrode configuration. Furthermore, a two-electrode based asymmetric supercapacitor using PPy/FeO@CVO as positive and graphene nanoplates (GNP) as negative electrodes and KOH soaked paper as separator reveals an outstanding energy density of 38.2 Wh/Kg (power density 700 W/Kg at 1 A/g) with amazing cycling stability (95% capacitance retention after 5000 cycles), suggesting great prospective of the ASC for high power device applications in modern electronic industries.

KEYWORDS: Hydrothermal, cobalt vanadium oxide hydrate, polypyrrole, specific capacitance, energy density. ACS Paragon Plus Environment

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1. INTRODUCTION The universal requirement of sustainable source of energy has instigated modern researchers to develop an alternative way to reproduce energy constantly via a cost effective and environmentally favorable pathway. Supercapacitors are the challenging energy storage device having combined properties of traditional battery and capacitors. The fast charging rate and high cyclic stability are the exclusive properties of supercapacitors. Conventionally two types of supercapacitors are used nowadays: electrical double layer capacitors (EDLCs) mostly consisting of porous carbonaceous electrode materials capable of store charge at the electrodeelectrolyte interface and pseudocapacitors typically comprised of redox active materials, e.g., transition metal oxide, hydroxides, conducting polymers which can store energy via a fast reversible redox reaction. From materials point of view, activated carbon, carbon nanotubes, graphene, etc. are well established materials for EDLC type electrodes1 while various transition metal oxides and hydroxides like MnO2, Co2O3, Co(OH)2, Fe2O3, NiO, Ni(OH)2,2 etc. are used for pseudocapacitive electrode preparation.3 Complex metal oxides,4 hydroxides,5 carbonates,6 pyrophosphates7 and ternary metal oxides, e.g., NiCo2O4,8 MnFe2O4,9 CoMoO4,10 etc. are recently utilized for high performance electrode fabrication. Layered transition metal vanadate based electrode materials have extensively been studied for rechargeable lithium ion battery application. The electrochemical properties of various layered transition metal vanadates solely depends on its preparation process, as well as, morphological features developed.11 In 2014, Wang et al. investigated nanosheets of cobalt vanadium oxide (Co3V2O8) having spectacular reversible capacity along with splendid rate performance for lithium ion storage.12 Very recently, Pang et al. demonstrated Co3V2O8 thin nanoplates having very high specific capacitance value along with superior cycle stability.13 Xiong et al. reported hexagonal Co2V2O7 microplatelets with nearly 100% capacity retention for highly reversible lithium storage.14 In 2014, Kong et al. studied Co3V2O8 and Ni3V2O8 based nanostructures for spectacular pseudocapacitive electrode materials.15 Recently, Kong et al. designed an asymmetric supercapacitor based on Co3O4/Co3(VO4)2 hybrid nanorods with satisfying capacitance and cycle stability.16 He also investigated nickel vanadate and nickel oxide ACS Paragon Plus Environment

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nanohybrid for excellent rate capability along with good cycle stability. The nanohybrid also shows a high power density accompanied with moderate energy density.17 However, there are very limited research works on layered nanostructured cobalt vanadium oxide as a sole electrode material for supercapacitor application reported till date. The layered type architecture provides better electrolytic ion transport owing to greater electrode-electrolyte contact zone while, cobalt and vanadium ions improves the ultimate electrochemical properties. Layered nanostructure also affords maximum diffusion of electrolyte ions and thereby facilitates the kinetics of ion transportation which is eventually reflected by their easy charge storage capability. Additionally, the free spaces in between the two consecutive layers accommodate the volume and dimensional changes during vigorous cycling conditions. The major negative aspects of these typical layered vanadates are low electron conductivity and structural collapse during vigorous electrolysis. In order to overcome these issues with enhanced electrochemical performance through synergistic interactions, researchers are prone to decorate or dope one type of metal oxide with another one having different morphological and electrochemical features. The selectivity of the metal oxides for decoration over the base materials solely depends on its morphology, crystal structure, affinity towards the base materials and the synergistic interaction among the two. In this aspect, Fe (III) and Fe (II) oxides are well established pseudocapacitive electrode materials having significant electrochemical properties18, 19 with enhanced conductivity. These promising eco-friendly oxides can be synthesized from a low cost and abundant source.20 Different iron based oxides can also be successfully decorated over various types of nano architectures with improved stability. It has been observed that reducing the size of iron oxide up to nano-level dramatically increases its ultimate electrochemical features and utilization efficiency of the electroactive material.21 In 2015, Xiao et al. studied the effect of morphology and the electrolyte solution upon the supercapacitive behavior of Fe2O3, and found outstanding capacitive performances of Fe2O3 nanosheets based electrodes.22 Tang and Meng et al. have successfully doped microspheres of MnO2 with homogeneously distributed Fe3O4 nanoparticles for enhanced electrochemical activity and cycle stability.18 Therefore, iron oxide decorated nano-architectures can be utilized for the ACS Paragon Plus Environment

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fabrication of advance electrode materials. Moreover, to reduce internal resistance (IR) and improve the power density of the electrode materials, one needs to develop a highly conducting path surrounding the base materials. This is due to the fact that the conductivity phenomenon minimizes the effective polarization and the mesopores interconnected through conducting channels ensures a high degree of electrodeelectrolyte interactions by reducing the ion transport path with facilitated electrolytic reaction kinetics. Currently, incorporation of metal oxides in conducting polymers is a very promising issue in the field of materials research to develop hybrid mesoporous nanocomposite. These polymers also exhibit a pseudocapacitive behavior based on the electrolytic environment and also achieves a moderate energy density under a wide working potential range. However, to achieve greater cycle stability, electrode materials should have adequate dimensional stability along with good packing to withstand the chemical stress applied during continuous cycling process. It can be achieved by decorating conducting polymer surrounding the base nanomaterials in a very cost effective way.23 Usually, polypyrrole (PPy), polyaniline (PANI), polythiophene (PTh), poly [ethylenedioxythiophene] (PEDOT) can easily be prepared and employed as light weight promising electrode materials with enhanced conductivity. Among those abundant conducting polymers, PPy has been nominated because of its several inherent qualities.24 Precisely, PPy can be synthesized directly in doped state by a facile and scalable process. It is highly conductive (ptype) having conductivity in the range of 10100 Ω.cm1.25 It has got a very high charge density, better chargedischarge rates,26 and extreme thermal

stability in air up to ~ 250 C. Das et al. reported transition metaldoped polypyrrole/multiwalled

carbon nanotubes nanocomposite with moderated capacitance and increased conductivity.26 Well oriented nanoarchitectures furnishes channels for ionic diffusion and thereby enhancing the overall performance with greater dimension stability. Furthermore, PPy has an intrinsic tendency to form ACS Paragon Plus Environment

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networklike arrangements and simultaneously crosslinks with each other in presence of Fe3+ ions.27 Formation of PPy network over FeO@CVO nanocanes simultaneously restricts the dimensional shrinkage and swelling during extreme cycling condition. It also restricts the dissolution of the constituent ion during vigorous chargedischarge process even in harsh electrolyte. The effect of breakage of polymer backbone on capacitive performance during electrochemical process is also reasonably less. The networklike architecture together with mesoporous features also provides facilitated electrolytic ion transport. In our present work, we have sought to combine the electrochemical properties of cobalt vanadium oxide hydrate (CVO), iron oxide and PPy together to achieve enhanced electrochemical performance. Densely oriented CVO nanocanes array were first prepared through a cost efficient hydrothermal method. Over the CVO nanocanes, Fe2O3 nanospheres were successfully decorated by using similar facile hydrothermal method. Finally, a robust, light weight PPy wrapped FeO@CVO mesoporous hybrid nanocomposite was designed by implementing in-situ oxidative polymerization of pyrrole. CVO is infrequently used as supercapacitor electrode material because of its low capacitance and inherent conductivity. This provides the scope for further investigation on CVO to widen its application in the area of capacitive material by improving its capacitance and inherent conductivity values. Thus, the novelty of the present work lies in use of CVO (commonly used in batteries) as efficient supercapacitor electrode material through decoration of CVO by Fe2O3 nanospheres, followed by wrapping it up with PPy that enhances the electrochemical properties and conductivity. The polymerization of pyrrole was carried out at a low temperature (05 ºC) in absence of acid. Presence of Fe3+ not only catalyzes the polymerization reaction, but also promotes formation of a gellike network of PPy surrounding the CVO nanocanes.27 Detailed analysis and characterizations reveals that CVO nanocanes with average diameter of 60~65 nm were successfully developed over which 5~10 nm globular shaped Fe2O3 together with small extent of Fe3O4 and FeOOH were decorated. Eventually, FeO@CVO was successfully wrapped with spherical PPy having diameters of 90~100 nm. Electrochemical results reveal that the hybrid composite deserves superior specific ACS Paragon Plus Environment

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capacitance and cyclic stability. Furthermore, an asymmetric supercapacitor (ASC) has been fabricated with PPy/FeO@CVO as positive and commercially available graphene nanoplates (GNP) as negative electrodes in presence of KOH soaked laboratory Whatman® 40 filter paper separator following a conventional twoelectrode configuration to enhance the ultimate power density and working potential window for supercapacitor application. The ASC with environmental friendly constituent electrodes also delivers moderate energy density with adequate stability. The uniqueness of the electrode material is accomplished as it has achieved a high energy and power density together with decent cyclic stability under vigorous cycling condition when assembled in twoelectrode configuration. 2. EXPERIMENTAL SECTION

2.1. Material Details. Cobalt (II) chloride hexahydrate [CoCl₂•6H₂O] (M.W.237.93 g/mole),

ammonium vanadate [NH4VO3] (M.W116.98 g/mole), iron nitrate [Fe(NO₃)₃·9H₂O] (M.W404

g/mole), pyrrole monomer (M.W67.09 g/mole) were purchased from Merck, Germany. Potassium hydroxide (KOH), ammonium persulfate [(NH4)2S2O8] (M.W. 228.18 g/mole) were purchased from

Loba Chemie Pvt. Ltd. India. Graphene nanoplates (Multilayer, carbon purity > 99.5%, D = 5 ∼ 25 μm,

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thickness 8 ∼ 10 nm) were purchased form J. K. Impex, Mumbai, India. All the reagents used were of

analytical grade purity and used without any further chemical purification. Deionized water having resistivity of 18 MΩ cm obtained from a JL-RO100 Millipore-Q Plus water purifier, was utilized in the experiments. 2.2. Synthesis of CoV2O6·2H2O nanocane arrays. Nanocanes of grass-like tangled CVO (CoV2O6·2H2O) was prepared by using a typical hydrothermal protocol for an extended time. In this typical synthesis process, 20 mL 1 M aqueous CoCl2 was mixed thoroughly with 20 mL 1 M aqueous NH4VO3 in a glass beaker under 20 min continuous stirring condition. The mixture was then shifted into a 50 mL Teflon sealed autoclave and heated in a muffle furnace at around 180 °C for 20 h. Initially, at the time of mixing under continuous stirring condition, no growth or precipitation was observed. After subsequent hydrothermal reaction, a brownish color precipitate was obtained which was then accumulated from the autoclave and washed with deionized water and ethanol for several times using centrifugation process. Finally the obtained product was dried at ~ 65 °C in a vacuum chamber for 24 h. The as prepared sample was termed as CVO. 2.3. Synthesis of iron oxide decorated CoV2O6·2H2O nanocanes. Iron oxide decorated CVO nanocane arrays was synthesized using similar hydrothermal procedure. In this process, 1 g of as-

prepared CVO was mixed thoroughly with 30 mL 0.05 M aqueous iron (III) nitrate [Fe(NO₃)₃]

solution by continuous stirring. The mixture was then shifted into a 50 mL Teflon sealed autoclave ACS Paragon Plus Environment

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and heated within a muffle furnace at around 180 °C for 6 h. Finally, the obtained precipitate was washed with deionized water for several times using centrifugation process and dried at ~ 65 °C in a vacuum chamber for 24 h. Hereafter, Iron oxide decorated CVO powder was termed as FeO@CVO. From the yield, we have calculated the wt. % of FeO and CVO in FeO@CVO. The obtained weight ratio of FeO:CVO  37.5:62.5. 2.4. Synthesis of polypyrrole wrapped iron oxide decorated CoV2O6·2H2O nanocanes. Spherical PPy wrapped FeO@CVO hybrid composite material was obtained by a simple low cost insitu chemical oxidative polymerization of pyrrole monomer in presence of FeO@CVO. At first, 0.3 mL pyrrole monomer was sonicated with 10 mL deionized water for 3 min to disperse it properly. In another beaker, 0.30 g FeO@CVO powder was sonicated with 20 mL deionized water for 20 min. The above two solution was mixed thoroughly by ~ 10 min of constant stirring under chilled condition (0~5 °C) in an ice bath to homogenize. Finally, 1 g of ammonium persulfate dissolved in 20 mL chilled deionized water was added to the above mixture drop-wise under stirring condition in order to initiate the polymerization. The color of the solution turned to olive green after some time. Ultimately, after 6 h of stirring, the obtained black precipitate was washed several times with deionized water and ethanol using centrifugation. The powder was then dried at 55 ºC for two days and reported as PPy/[email protected] From the yield, the calculated weight ratio of PPy:FeO@CVO  49:51. 2.5. Preparation of working electrode. The working electrode was prepared by mixing active materials with carbon black and polyvinylidine fluoride (PVDF) in a ratio of 8:1:1 (w/w/w) in presence of minute N-Methyl-2-pyrrolidone (NMP) to form a paste. The prepared homogenous paste was then slowly casted on nickel foam (Ni foam) and dried completely in open air for 24 h. Then, the specimens (Ni foam coated individually with PPy, CVO, FeO@CVO, PPy/FeO@CVO and GNP) were shaped in 1.5×1.5 cm2 dimension.28 Ni foam was utilized as current collector. Finally, the entire electrochemical measurements i.e. galvanostatic chargedischarge including cycle stability, cyclic voltammetry and electrochemical impedance spectroscopic studies (EIS) were conducted by employing a three-electrode cell setup using Biologic SP150 instrument with 1M aqueous KOH ACS Paragon Plus Environment

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solution. The electrochemical measurements of the assembled ASC have been accomplished using a two-electrode configuration with KOH soaked laboratory Whatman® 40 filter paper on the same instrument. The detailed fabrication process has been depicted schematically in Figure 1 and the electrochemical approach was thoroughly described.

Figure 1. Schematic representation for the fabrication of PPy/FeO@CVO composite electrode and PPy/FeO@CVO//GNP ASC device. 3. CHARACTERIZATIONS Powder XRay diffraction (XRD) studies were performed using XPert PRO diffractometer [PANalytical, Netherland] having monochromatic Cu Kα radiation (λ=0.15418 nm) at a scan rate of 0.5o/min. Field emission scanning electron microscopic (FESEM) studies were performed by ACS Paragon Plus Environment

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applying an operating voltage of 5 kV in Carl ZeissSUPRA 40 (Oberkochen, Germany) under vacuum (104 ─106 mm Hg). HighResolution Transmission Electron Microscopic (HRTEM) studies were performed (JEM─2100, JEOL, Tokyo, Japan) at a working voltage of 200 kV. Quantitative elemental detection was performed using Energy Dispersive X–ray Spectroscopy (EDS) attached with FESEM and TEM. The surface topology of the electrode materials were detected by Nanonics Multiview 1000TM (Israel) SPM system with a quartz optical fiber tip (diameter = 20 nm, spring constant = 40 N m1) in tapping mode.29 Raman spectroscopic analysis was conducted on Raman triple spectrometer (T64000, HORIBA Jobin Yvon, France) with a wavelength of 632.8 nm ArKr mixed ion gas laser coupled with synapse detector.23 Fourier Transformed Infrared (FTIR) spectroscopic analysis was performed using Nexus 870 FTIR instrument (Thermo Nicolet) within the range of 4000 ─ 400 cm1. Xray photoelectron spectroscopic measurement was carried out using a PHI 5000 Versa Probe II scanning Xray photoelectron spectrometer (XPS) [Al Kα source ~1486 eV].30 Nitrogen adsorption isotherm was assessed at 77 K with a Quantachrome ChemBET TPR/TPD analyzer. The sample was degassed at 100 ºC for 3 h under vacuum condition before performing nitrogen adsorption kinetics. The effective exposed surface area was estimated by employing Brunauer–Emmett–Teller (BET) model while the pore size and volume was determined by Barrett–Joyner–Halenda (BJH) method.23 Electrochemical measurements of the as-prepared electrode materials were carried out using a typical three-electrode system. In a three-electrode cell, all the electrode materials coated Ni foams were employed as working electrode, individually, while platinum electrode (1x1 cm2) and saturated calomel electrode were used as counter and reference electrode, accordingly. Electrochemical measurements of the ASC were executed using a two-electrode configuration. In a two-electrode cell setup, PPy/FeO@CVO was employed as positive and GNP as negative electrode, with KOH soaked Whatman® 40 filter paper operable within 01.4 V wide potential window.

4. RESULTS AND DISCUSSION


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To explore the crystal structures of the as-prepared electrode materials, WAXD analysis were conducted by mounting samples on a transparent glass fibre plate and placed them in Xray diffractometer instrument. Figure 2(ad) represents the Xray diffractogram used to determine the crystal structure and lattice parameters. The obtained diffractogram ensures the formation of CoV2O6·2H2O nanocanes as it coincides well with the JCPDS card no. 41-0420. High intensity peaks appears at 2θ 17.6o, 22.02o, 22.4o and 27.6o corresponds to (101), (102), (022) and (122) crystal planes while other characteristic low intensity peaks at 2θ 14o, 26.16o, 33.17o, 42.8o and 52.9o stand for (020), (031), (201), (134) and (243) crystal planes, respectively (Figure 2a). The diffractogram of FeO@CVO (as shown in Figure 2b) elaborates the formation of mixed iron oxides over CVO nanocanes surface. Along with most of the CVO signatures, the diffractogram exhibits mixed moderate to low intensity peaks at 2θ 13.6o, 31.5o and 48.45o corresponds to (006), (113) and (127) plane of Fe2O3 (JCPDS 0401139) While, signature at 2θ 30.04o indicates the presence of Fe3O4 (220) planes (JCPDS 010890950). Furthermore, diffraction peaks at 2θ 29o and 50.75o stands for FeOOH (003) and (005) planes (JCPDS 000461315), respectively. This result well depicts the formation of mixed iron oxide (solely Fe2O3 with small existence of Fe3O4 and FeOOH) over CVO nanostructure. As observed from the XRD profile of FeO@CVO, it can be inferred that presence of iron oxides marginally shifts certain CVO peak positions (as compared to Figure 2a) indicating better doping and good distribution of FeO over CVO nanocanes. The diffractogram of the as-prepared PPy/FeO@CVO (Figure 2c) exhibits a broad peak at a 2θ  2528o (Inset) supporting the presence of significant amount of PPy (104) planes along with most of the acquired FeO@CVO signatures. The high intensity peaks of FeO@CVO appeared over less intense broad PPy (104) hump. It has been observed that the signature of PPy has been suppressed in appearance of highly intense FeO@CVO peaks. The obtained result suggests amorphous PPy network successfully wrapped over FeO@CVO nanocane arrays. The XRD pattern of pure PPy network is referenced in Figure 2d.31 Probable intermolecular interaction between PPy moiety and FeO@CVO as proposed in Figure 1 may be the


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possible reason behind a marginal shifting of the PPy peak position in PPy/FeO@CVO in contrast to pure PPy.

Figure 2. XRD patterns of (a) CVO, (b) FeO@CVO, (c) PPy/FeO@CVO and (d) pure PPy. The probable 3D crystal structure of CVO (as shown in Figure S1, Supporting information†) was designed by taking the reference of the diffractogram represented in Figure 2a. The structure consists of four atoms i.e. cobalt, vanadium, oxygen and hydrogen. During crystallization, it acquires orthorhombic crystal form with a pnma space group (space group no. 62). The average crystallite sizes of CVO was calculated by using Scherrer equation (Equation-1) for four most intense peaks appeared at various positions with 2θ value of 17.6o, 22.02o, 22.4o and 27.6o (as tabulated in Table S1†) which is further supported by high resolution transmission electron microscopic studies. CVO d hkl 

0.9   180    cos 

Equation-1

Where λ denotes the wave length of Cu Kα, β = FWHM and θ is the Braggs angle. The morphological characteristics were ascertained through field emission scanning electron microscopic (FE-SEM) studies and represented in Figure 3(ah). The FE-SEM images depicted in ACS Paragon Plus Environment

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Figure 3a and b exhibits the cane-like structures of CoV2O6·2H2O. Two or more single nanocanes are intermittently assembled together in a random array to form a bunch of entangled nanocanes. The average diameter of single CVO nanocanes are 60~65 nm and the surfaces are moderately smooth. The diameter of CVO nanocanes is not consistent throughout the length (Figure 3b) because of the shrinkage in structure during recrystallization.13 Figure 3c, d depicts FeO@CVO where the presence of small amount of iron oxide influences formation of a network-like arrangements of CVO nanocanes with increase in pore size and ditches (Figure 3c). The spherical type iron oxide nanoparticles with diameters 5~10 nm are deposited throughout the whole upper surface of CVO nanocanes (as shown in Figure 3d). The morphological features of PPy/FeO@CVO nanocomposite as represented in Figure 3e, f illustrates that all the entangled nanocane arrays were well wrapped with granular type PPy network with average diameter in nanometric scale length (Figure 3e). However, some non-uniformity in the coating was observed due to less amount of PPy. The wrapping of PPy over FeO@CVO suggests significant interaction between the two. It is to be specified that the quantity of pyrrole monomer taken was reasonably less (0.3 mL) at the beginning of the polymerization which is imitated by restricted non-uniform coating of PPy over CVO nanocane surfaces. If the quantity of pyrrole monomer surpasses beyond its optimization limit (0.3 mL), the wrapping will be more widespread and extensive causing complete coverage of CVO nanocanes by PPy which restricts electrochemical activities. Figure 3g, h depict the morphological features of pure PPy prepared by following the same procedure without using FeO@CVO nanohybrid filler in order to check the effect of filler in the morphology of the polymer. PPy shows similar granular or spherical type morphology with an increased average diameter (400430 nm) (Figure 3h inset). In the reverse way, the decrease in the granular size of PPy in presence of FeO@CVO with respect to pure PPy indicates strong interaction between PPy and nanohybrid filler. These strong interactions supposed to be in between electron deficient H atom directly attached to N atom in a PPy moiety and highly electronegative O atom of CVO. The porous nature of the nanocomposite furnishes greater surface area suitable for faradic redox reaction and the frequent transportation of the electrolyte ions through electrodeelectrolyte interfaces. ACS Paragon Plus Environment

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Figure 3. FE-SEM micrographs of (a) and (b) CVO nanocanes, (c) and (d) FeO@CVO, (e) and (f) PPy/FeO@CVO, (g) and (h) pure PPy. The elemental mapping of the as-prepared electrode materials, as shown in Figure S2(ac)† depicts a homogeneous distribution of the constituent elements. The presence of Co, V, O (Figure S2a), small amount of Fe along with CVO (Figure S2b) and C and N of PPy (Figure S2c) can be corroborated. Energy dispersive X-ray line pattern (Figure S3†) also demonstrates the similar information. The morphological changes obtained after 3000 consecutive chargedischarge cycles were also investigated (S1†). High resolution transmission electron microscopic (HRTEM) and atomic force microscopic analysis (as shown in S2†) also support the features obtained from FE-SEM studies. Figure 4(a-i) represents the high resolution TEM images of the as prepared electrode materials. As can be seen (Figure 4a, b), there was an obvious formation of CVO nanocanes having moderately smooth surface. The nanocanes assembled together in a random array (Figure 4a) and its diameter varies in between 5565 nm (Figure 4b).32 Selected area electron diffraction pattern (SAED) as illustrated in Figure 4c exposes the polycrystalline nature of CVO. More specifically, the grain boundaries are present in much larger portions than the single crystal which conveys easy transport of electrolyte ions.17 Figure 4d, e shows the TEM images of FeO@CVO nanohybrid, where 510 nm particles of spherical iron oxides were found to decorate over the surfaces of nanocanes. The SAED pattern ACS Paragon Plus Environment

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depicted in Figure 4f also exposes the polycrystalline nature of CVO in FeO@CVO. The signature of iron oxide [020] and CVO [122] planes as detected from SAED pattern further confirms the presence of iron oxide. Figure 4g and h depicts the obtained low and high magnification TEM images of PPy/FeO@CVO nanocomposite, accordingly. The presence of PPy can be easily determined from both the images and it has been seen that vanadate nanocanes are appropriately wrapped/covered with amorphous granular PPy. Some regions surrounding the nanocanes were evident where the PPy forms agglomerates. This is perhaps due to the presence of Fe3+/Fe2+ which promotes interconnection among neighboring PPy chains. Figure 4i indicates the TEM image of pure PPy prepared without using FeO@CVO. PPy shows similar granular and agglomerated morphology which affirms FESEM results.

Figure 4. TEM images of (a) and (b) CVO, (d) and (e) FeO@CVO, (g) and (h) PPy/FeO@CVO, (i) pure PPy. (c) and (f) represents the SAED patterns of CVO and FeO@CVO, accordingly. Raman spectroscopy (as elaborated in S3†) and Fourier transformed infrared (FTIR) spectroscopic analysis (S4†) were carried out to determine the molecular structure, interactions and bond information of the as-prepared electrode materials.


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X-ray-photoelectron-spectroscopic (XPS) analysis was employed to examine the surface chemical composition and the oxidation states of the elements present in CVO and FeO@CVO. Figure 5a demonstrates the overall XPS survey scan plot and Figure 5(be) depicts the selected area depth profile scan plots of FeO@CVO. The Co 2p, V 2p, Fe 2p, O 1s and C 1s regions are properly identified from the survey spectrum. The spectrum was taken with reference to aliphatic carbon having a binding energy of 283.9 eV. Gaussian fitting method has been utilized for all the depth profile scan plots of FeO@CVO (Figure 5be) in order to determine the exact peak position and the binding energy gap between various peaks of a particular element present. The Co 2p spectrum of FeO@CVO as represented in Figure 5b shows two separate peaks of 2p3/2 at 780.7 eV and 2p1/2 at 796.6 eV, with a binding energy gap of 15.9 eV between 2p3/2 and 2p1/2 peaks. The oxidation states of cobalt present in the as prepared material are solely related to the energy gap between the main Co 2p peaks and the satellite peaks.14, 33 The binding energy difference between the Co 2p3/2 main peak at 780.7 eV and the satellite peak at 786.1 eV is around 5.6 eV. Further a second satellite peak was identified at 6.6 eV above the Co 2p1/2 peak. All the specified signatures of Co 2p depict that cobalt present predominantly in Co(II) state in the compound with trace amount of Co(III).34 Similarly, using Gaussian fitting method; four peaks have been identified from the V 2p spectrum (Figure 5c). V 2p3/2 and 2p1/2 peaks appear at a binding energy of 516.7 eV and 523.7 eV, respectively. The binding energy gap (~7 eV) between the two main peaks of V elucidates the presence of V(5+) state. The appearance of satellite peak at 517.5 eV, which is 0.8 eV higher than that of V 2p3/2 peak (516.7 eV), also supports the presence of V(5+) state. For O 1s spectrum (Figure 5e), one broad peak appears at a binding energy of 530.1 eV suggesting the presence of oxygen in the form of oxide within the as prepared FeO@CVO. Appearance of a less intense peak at 531.2 eV suggests the presence of small amount of hydroxyl moiety in the form of water within the crystal structure, consistence with the XRD results.14 The Fe 2p spectrum obtained for as prepared FeO@CVO (Figure 5d) shows two separate peaks of Fe 2p3/2 and Fe 2p1/2 at a binding energy of ~710.6 eV and 723.8 eV, respectively. It is already reported in the literature that the binding energy of Fe 2p3/2 appears at ~ 709 eV for Fe2+ and 711 eV for Fe3+.30, 35 The ACS Paragon Plus Environment

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above mentioned peaks for Fe 2p appeared in FeO@CVO support the presence of iron both in Fe (II) and Fe (III) states. The main peak of Fe 2p3/2 can be deconvoluted into two parts having a binding energy of 709.9 eV and 712.3 eV, respectively. The satellite peak appears at 717 eV describes the presence of Fe(III) state in the form of Fe2O3.36 Furthermore, presence of a small peak at 529.4 eV is more likely related to the formation of very minute (almost negligible) quantity of FeOOH species. The XPS spectrum of as-prepared CVO (as shown in Figure S8†) also confirms the presence of Co(II)/Co(III), V(5+) and O2 states. All the revealed results unambiguously suggest the formation of CoV2O6·2H2O, which was decorated with mixed iron oxide (mainly Fe2O3 with some extent of Fe3O4). This finding is also in good agreement with the FESEM and TEM investigations. Henceforth, the mixed iron oxide is referred to as FeO in a general way throughout the manuscript.

Figure 5. X-ray photoelectron (XPS) spectra of the as-prepared FeO@CVO: (a) Overall survey spectrum, (b) Co 2p, (c) V 2p, (d) Fe 2p, and (e) O 1s XPS spectrum.


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XPSEDAX analysis (as shown in Table 1) illustrates the presence of significant amount of Fe along with Co, V and O in as-prepared FeO@CVO composite. Materials

Elements Present [Atomic %] Co

V

Fe

O

1. CVO (Figure S8 )

15.40

23.43

0.00

61.16

2. FeO@CVO

8.61

25.92

6.73

58.74

†

Table 1. XPSEDX analysis of CVO and FeO@CVO. Brunauer–Emmett–Teller (BET) analysis was executed for PPy/FeO@CVO hybrid electrode material to determine the specific surface area and the pore size distributions. Figure 6a represents the nitrogen adsorptiondesorption isotherm of PPy/FeO@CVO at a temperature of 77 K. The isotherm follows TypeIII pattern having H3 hysteresis loop mainly attributed to the presence of mesopores within the as-prepared hybrid composite electrode material. It has been found that the BET surface area (210.04 m2g 1) of PPy/FeO@CVO was significantly high. This high value is attributed to the wrapping of FeO@CVO by PPy which significantly enhances the overall exposed specific surface area of the ultimate composite. Figure 6b represents the pore size distribution of PPy/FeO@CVO. Paramount mesoporous nature of the as-prepared hybrid composite electrode material can be clearly understood with a pore diameter of 3.146 nm. It has already been reported that the pore diameter of the mesoporous materials should lie within the range of 250 nm. The presence of mesopores creates easy access for the transportation of the electrolyte ions and hence, faster rate of charge transfer.23,

37

Overall pore volume was measured to be 1.215 cc g 1 at P/Po= 0.997, as calculated using BJH method.


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Figure 6. N2 sorption isotherm (a) and pore size distribution (b) of PPy/FeO@CVO nanocomposite measured at 77 K. To investigate the effect of elemental composition and morphology on the electrochemical properties, complete electrochemical studies of CVO, FeO@CVO and PPy/FeO@CVO were carried out through cyclic voltammetry (CV), galvanostatic chargedischarge (GCD) and electrochemical impedance spectroscopy (EIS) using 1 M aqueous KOH electrolyte. Prior to the electrochemical measurements, all the active electrodes were charged and discharged at 2 A/g current density for ~ 100 cycles until a steady capacitance value was obtained. Usually, the active electrode materials show some variations in their specific capacitance values at the beginning. This may be due to incomplete activation or improper wetting of the electrode materials in the electrolyte solution initially. After 100 cycles the materials shows specific capacitance values with a little or no variations. This indicates that after ~ 100 cycles the electrode materials become completely activated and there are convenient electrodeelectrolyte interactions along with proper wetting of the electrode surface. Finally, CV, GCD and EIS were performed after achieving the steady state.3 Cyclic voltammetry profiles of CVO, FeO@CVO, PPy/FeO@CVO and pure PPy are represented in Figure 7(ad) with varying scan rates of 2, 5, 10, 30, 50, and 100 mV/s within a potential window of 0.1 to 0.5 V. The non-rectangular CV curves of CVO (Figure 7a) explore its pseudocapacitive nature in 1 M aqueous KOH electrolyte. With increasing the scan rate from 2 mV/s to 100 mV/s, the anodic


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peak shifts towards higher positive potential values and the cathodic peak shifts toward lower negative potential values accordingly. This could be due to increased overpotential of the electrode material. A plausible quasi-reversible electron transfer redox reaction mechanism can be predicted as follows:38 Co2+ + 3OH = CoOOH + H2O + e

(I)

CoOOH + OH = CoO2 + H2O + e

(II)

In alkaline medium, equilibrium exists between Co(II) and Co(IV) states for CVO. The CV curves of FeO@CVO (Figure 7b) also exhibit similar redox peaks indicating its pseudocapacitive nature, where, along with corresponding signatures of CVO, a new broad hump with higher current response is also noticed to appear signifying the presence of iron oxide (Fe2O3 and Fe3O4) nanoparticles19 decorated on the surface of CVO [as observed in FE-SEM and TEM]. The plausible redox reaction mechanism for FeO@CVO can be described as follows: Fe3+ + Co2+ = Fe2+ + Co3+ (III) Figure 7c demonstrates the CV curves of the as-prepared hybrid PPy/FeO@CVO nanocomposite. The pseudoconstant rate of electrochemical redox reaction and favorable electrodeelectrolyte interaction in the hybrid composite electrode can be resolved by the appearance of broad redox peaks. Diffusion of electrolyte within the inner pores of electrode materials through electrodeelectrolyte interface can also be easily depicted by the current response which gradually increases with increasing the scan rate.39 It has been perceived that at any scan rate, the calculated area under the CV plot for PPy/FeO@CVO is highest amongst the other two, indicating a high specific capacitance value. Figure 7d demonstrates the CV plots of pure PPy where a slight anodic signature appears at ~ 0.37 V and cathodic signature at ~ 0.12 V. The calculated area under the CV curve of pure PPy is very less compared to all the other as-prepared electrode materials displaying a low capacitance value within the experimental potential window. Thus, it can be concluded that incorporation of CVO within PPy matrix enhances the specific capacitance values as well as electrochemical properties. Figure 7e represents the CV plots of all the electrode materials altogether at 2 mV/s scan rate for easy assessment. The current response of PPy/FeO@CVO (III) is greater than both FeO@CVO (II) and


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CVO (I), suggesting PPy/FeO@CVO as potential candidate for efficient electrode materials. Inset of Figure 7e illustrates the enlarged view of all the CV curves obtained at lowest scan rate for better clarity.

Figure 7. Cyclic voltammetry plots of (a) CVO, (b) FeO@CVO, (c) PPy/FeO@CVO, (d) pure PPy at different scan rates and (e) CV plot altogether at 2 mV/s scan rate (I. CVO, II. FeO@CVO, III. PPy/FeO@CVO, IV. PPy). The specific capacitance of all as-prepared electrode materials were calculated from their respective CV plots using Equation2, as described below: 10, 23 V2

 i(V )dv Specific capacitance (CS ) 

V1

(V2  V1 ) m

(Equation2)

Where, V1 and V2 represent the lower and the upper potential limits, i indicates the current response, ν indicates the scan rate and m is the effective mass of the electrode material. The area under the CV curves will be exactly equal to the numerator part of the above equation.3 Table 2 summarizes the calculated specific capacitances of the electrode material at different scan rates. The maximum specific capacitance value obtained for PPy/FeO@CVO nanocomposite was 1109 F/g at a scan rate of


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2 mV/s which is much larger than the calculated specific capacitance values for CVO (627 F/g) and FeO@CVO (931 F/g) acquired at same scan rate. It has been observed that with raising the scan rate, the specific capacitance value decreases gradually10 (as shown in Figure S9a†). The obtained CV curves of PPy/FeO@CVO shows a high area at all scan rates compared to the other two materials solely due to increase in the conductivity and pseudocapacitive behavior in presence of conducting PPy. As mentioned earlier, 0.3 ml of pyrrole monomer was used as optimum amount during preparation of the nanocomposite. Beyond this optimum amount of pyrrole, the amount of PPy becomes more leading to complete coating of FeO@CVO nanocanes rather than wrapping by PPy. In that circumstance electrolyte will not be able to penetrate in CVO phase and ion transportation will be prohibited. Hence, capacitance will be decreased with increase in the amount of pyrrole. Materials CVO [mass-0.0135g] FeO@CVO [mass-0.0123g] PPy/FeO@CVO [mass-0.0116g] PPy (Pure) [mass-0.0109g]

Specific capacitance [F/g] at different scan rates 2 mV/s 5 mV/s 10 mV/s 30 mV/s 50 mV/s 100 mV/S 627 542 465 334 266 172 931

842

721

601

522

369

1109

1004

887

762

655

500

324

260

213

153

106

71

Table 2. Values of specific capacitance with scan rates for all the as-prepared materials. Galvanostatic chargedischarge (GCD) measurement is the most authentic method to determine the specific capacitance of any electrode material under constant current density. Specific capacitance values for the electrode materials were measured from the GCD plots with the help of Equation3, as follows:10, 23 Specific capacitance (Cs) =

i  t m  v

(Equation3)

Here, Cs is the calculated specific capacitances in F/g, (i/m) represents the current density in A/g, ΔV and Δt are the applied potential window in volts and discharging time in seconds, respectively. The GCD measurement of CVO, FeO@CVO, PPy/FeO@CVO and pure PPy has been conducted within a potential range of 0.1 to 0.4 V at various current densities using 1 M KOH and represented in Figure ACS Paragon Plus Environment

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8(ad). All the obtained chargedischarge curves show nonlinear behavior which clearly elaborates the pseudocapacitive nature of the as-prepared materials under the applied potential window. Figure 8a and b depict the GCD curves of CVO and FeO@CVO at different current densities, respectively. The specific capacitance values obtained for CVO and FeO@CVO were 708 F/g and 968 F/g, respectively, at 1 A/g current density. The GCD plots of PPy/FeO@CVO (as shown in Figure 8c) display longest discharging time amongst the other electrode materials and hence, contribute highest specific capacitances of 1202 F/g at 1 A/g. The synergistic interactions between conducting PPy and pseudocapacitive FeO decorated CVO are the key factor to enhance the electrochemical performance of the as-prepared hybrid composite. Moreover, PPy/FeO@CVO being more electrically conductive shows least IR drop during discharging amongst all the others. Presence of mesoporous PPy also increases the overall effective surface area for convenient electrodeelectrolyte interaction and thereby facilitating the redox process during constant charge-discharge cycle. Figure 8d depicts the GCD curves of pure PPy. The calculated specific capacitance value for pure PPy was 258 F/g at 1 A/g. Figure 7e represents GCD curves of all as-prepared electrode materials collectively at 1 A/g current density.


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Figure 8. GCD plots of (a) CVO, (b) FeO@CVO, (c) PPy/FeO@CVO, (d) pure PPy at different current densities and (e) altogether at a 1 A/g current density. The calculated specific capacitances of the as-prepared materials at different current densities were tabulated in Table3. It has been observed that with increasing the current density, the specific capacitance value decreases40 (as shown in Figure S9b†). This is perhaps due to lesser diffusion of the electrolyte ions within the pores of the electrode materials and unavailability of the redox sites at high current densities. Materials CVO [mass-0.0135g] FeO@CVO [mass-0.0123g] PPy/FeO@CVO [mass-0.0116g] PPy (Pure) [mass-0.0109g]

Specific capacitance [F/g] at different current densities 1 A/g 1.5 A/g 2 A/g 3 A/g 708 510 424 318 968

642

440

360

1202

831

556

456

258

189

76

3.6

Table 3. Values of specific capacitances with current densities for the electrode materials. Furthermore, a brief comparison of our acquired results with other nanostructured morphologies reported elsewhere has been tabulated in Table S2†. The cycle stability experiment for PPy/FeO@CVO at a constant 1 A/g current density using 1 M KOH electrolyte exhibits maximum retention (~96.5%) in specific capacitance after 3000 successive GCD cycles in comparison to CVO (~80%), FeO@CVO (~88%) and pure PPy (~60%) as represented in Figure 9. This unprecedented shifting of the cyclic stability to a remarkably higher value (~96.5%) in PPy/FeO@CVO may be explained in terms of strong interactions between PPy and FeO@CVO (Figure 1) and stability of PPy matrix. Here, the presence of Fe2+/Fe3+ ions is expected to produce an interconnected network structure in PPy matrix24 that restricts the breakage of polymer backbones during vigorous charging and discharging.23


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Figure 9. (a) Cyclic stability plots of CVO, FeO@CVO, PPy/FeO@CVO, and pure PPy at 1 A/g. In order to explore various resistances offered by the electrode materials, electrochemical impedance spectroscopic (EIS) studies were performed within a frequency range 1 MHz

100 mHz.

The EIS analysis and the corresponding Nyquist plots (imaginary component of the impedance (–Zimg) vs. real component of impedance (Zreal)) were exhibited in Figure 10. Inset of Figure 10 depicts the impedance behavior of the electrode materials at high frequency region for the same Nyquist plot. The corresponding fitted circuit diagram was also represented in the upper inset of Figure 10. The equivalent circuit exhibits various resistance characteristics, such as, (1) Solution resistance (Rs) which is a direct measure of the resistance of the substrate along with the electrodeelectrolyte interface resistance; (2) Charge transfer resistance (Rct); (3) Warburg behavior of the electrode materials and (4) Constant phase element (CPE or Q). By taking into consideration of all the above four resistance characteristics with a double layer capacitance (Cdl) combined in parallel, the overall circuit resistance can be represented mathematically as Rs+ Q/(Rct+W) + Cdl/Rct. The Nyquist plots for the individual as-prepared electrode materials shows similar kind of behavior with a starting semicircular zone at high frequency region followed by a steeper linear profile at lower frequency zone. Charge transfer resistance (Rct) of the electrode materials can be determined by measuring the diameter of semicircular region at high frequency. The initial intersection point of the semicircular curve with the real impedance axis at high frequency region determines solution resistance (Rs) of the materials. The ACS Paragon Plus Environment

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diffusion phenomenon i.e. Warburg behavior of the electrode materials can be determined from the slope of the steeper linear part at low frequency zone. More ion diffusion can be reflected with a high slope value of the linear part. If the steeper line appears exactly parallel to the imaginary impedance axis then it can be concluded that the electrode materials behaves like an ideal capacitors.23 The solution resistances (Rs) of all as-prepared electrode materials were calculated to be 1.24, 0.73, 0.51 Ω for CVO, FeO@CVO, PPy/FeO@CVO, respectively. The charge transfer resistance (Rct) of PPy/FeO@CVO shows the lowest value of 2.8 Ω as compared to CVO (6.20 Ω) and FeO@CVO (3.81 Ω) indicating greater exposure of PPy/FeO@CVO hybrid nanocomposite towards faradic redox reaction. The presence of frequent mesopores plays a crucial role for this lowest Rs and Rct values of PPy/FeO@CVO amongst the three. The Warburg behavior of the electrode materials depicts that PPy/FeO@CVO has a higher Warburg impedance value than CVO and FeO@CVO and henceforth greater diffusion of the electrolytic ions as compared to others. Presence of conducting PPy and iron oxides in PPy/FeO@CVO makes the entire composite more conducting in nature along with prone towards electrolytic ion transportation at the electrodeelectrolyte interface.12 It is noteworthy, none of the electrode materials exhibits ideal capacitive behavior (as discussed earlier). Hence, a constant phase element (CPE) has been introduced in the equivalent circuit diagram instead of purely capacitive element to achieve better fitting of the obtained values with the experimental results.26


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Figure 10. Nyquist plots of CVO, FeO@CVO and PPy/FeO@CVO. Impedance plots at high frequency region are pictured in the right inset. The equivalent electrical circuit fitted to the corresponding Nyquist plots is visualized on the upper inset. The superior electrochemical and capacitive behavior delivered by as-prepared PPy/FeO@CVO hybrid composite electrode is absolutely due to synergistic interaction between granular PPy and FeO@CVO nanocanes. Co and Fe ions offer improved electrochemical and pseudocapacitive response while granular PPy macromolecules gives exceptionally high cyclic and dimensional stability even in harsh chemical environments. Ni foam substrate was used as a current collector. The electrochemical performance of GNP coated Ni foam as negative electrode has been explored with 1 M aqueous KOH electrolyte (S5†). In accordance with the acquired results, an asymmetric supercapacitor (ASC) based on twoelectrode configuration has been fabricated by employing PPy/FeO@CVO as a possitive and GNP as negative electrodes in presence KOH soaked Whatman® 40 filter paper as a separator. The porous separator soaked with electrolyte solution restricts the contact between the two electrodes while permits the electrolytic ion transport. The EDLC type GNP and pseudocapacitive PPy/FeO@CVO electrodes collectively contributes to the ultimate capacitive performance and furnish better ACS Paragon Plus Environment

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energy/power density to the assembled ASC. The fabricated ASC can offer combined properties of supercapacitors and batteries with a benefit of wide working potential range. Prior to the electrochemical measurements, the ASC has been chargeddischarged at 2 A/g for ~ 100 repetitive cycles in order to achieve a steady capacitance value. Thereafter, complete electrochemical characterization has been executed within a wide operating potential range of 01.4 V. The charge balance between the two constituent electrodes for a two-electrode configuration follows the relation q+= q, where q+ and q are the charge of the positive and negative electrode, accordingly. The charge stored within the electrode can be calculated by Equation4, as follows:

q  C  E  m

(Equation4)

Where, C, E and m are the specific capacitance in (F/g), working potential range (V) and mass of the active electrode material (g). The necessary mass balancing equation for the fabrication of an ASC can be expressed as follows:41

m C   E   m C   E 

(Equation5)

The specific capacitance values obtained for PPy/FeO@CVO and GNP electrodes has been introduced into Equation5 along with their operating voltage window to calculate the exact mass ratio. The obtained mass ratio (m+/m) is ~ 0.257 in the ASC. Figure 11a demonstrates the CV plots of the ASC obtained at various scan rates of 2, 5, 10, 30, 50 mV/s. The enlarged view (as shown in the inset) demonstrates outstanding capacitive nature of the ASC. The combined electrochemical response (Faradic and EDLC) of the two constituent electrodes can be easily predicted. The nature of the CV curves remains almost similar at all scan rates, demonstrating outstanding capacitive performance. The specific capacitance values at different scan rates are calculated using Equation2, where ‘m’ signifies total mass of the constituent electroactive material. The maximum specific capacitance value obtained from the CV profile of PPy/FeO@CVO//GNP ASC is ~ 119.7 F/g at 2 mV/s scan rate. Figure 11b illustrates the GCD plots of the ASC obtained at various current densities. The shape of the GCD profile with a very tiny plateau region eventually demonstrates admirable capacitive response of the ACS Paragon Plus Environment

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ASC. The specific capacitance values at different current densities are calculated applying Equation3, where ‘m’ is the total mass of the electroactive material. A maximum specific capacitance of 140.5 F/g is obtained at 1 A/g which remains up to ~ 83 F/g at 3 A/g. The energy density (E) and power density (P) of the ASC was calculated by using the following Equation6, 7, respectively.42, 43

E ASC 

1  C ASC  ( V ) 2 2

PASC  E ASC T

(Equation6) (Equation7)

Here, EASC denotes the energy density in Wh/Kg, ΔV stands for the voltage drop during discharge in (V), CASC is the total specific capacitance of the ASC (F/g), PASC signifies power density in W/Kg, T represents the discharge time (s). Figure 11c illustrates the Ragone plot of the fabricated ASC based on twoelectrode configuration obtained at a voltage window of 01.4 V. The ASC exhibits an outstanding energy density of ~ 38.2 Wh/kg at a power density of 700 W/kg (@1 A/g) and surprisingly, the energy density remains at ~ 22.5 Wh/Kg even at an elevated power density of ~ 2099.2 W/Kg (@ 3A/g). Moreover, the ASC exhibits much higher energy density than that of grapnene, CNT based symmetric supercapcitors reproted elsewhere, e.g. CNT//CNT symmetric supercapacitor (< 10 Wh/Kg),44 graphene//graphene symmetric supercapacitor (< 10 Wh/Kg).45 The ASC also reveals superior power density than other vanadium based electrode materials or devices as reported in the literature; e.g., Kong et al. studied the electrochemical performance of an ASC based on AC//Co3O4/Co3(VO4)2 with an energy density of 38 Wh/kg and a power density of 275 W/kg.16 In 2014, they inspected the capacitive performance of Co3O4–Ni3(VO4)2@Ni foam with an energy density of 31.2 Wh/kg at 0.5 A/g.46 Wu et al. reported the capacitive behavior of PPy@V2O5//AC with an energy density of 42 Wh/kg. They also studied V2O5//AC based ASC with an energy density of 23 W h/kg.47 In 2015, Liu et al. reported the capacitive behavior of Ni2P/Co3V2O8//AC based ASC with an energy density of 40.2 Wh/kg.48 A table containing a comparison of energy and power density of our as-fabricated ASC with the best results reported in the literatures has been explored in Table S3†. An immense energy and power density exhibited by our fabricated ASC reveals its great applicability ACS Paragon Plus Environment

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in power device arena. An outstanding cycling performance over 5000 consequitive chargedischarge cycle has been represented in Figure 11d. The GCD cycling stability experiment has been carried out within a potential window of 01.4 V and by applying 1 A/g fixed current density. An exceptional cycling stability with 95% specific capacitance retention after 5000 consequitive GCD cycles has been perceived. The continious GCD profile of the ASC device upto ten consequitive cycles measured at 1 A/g current density is illustrated in the inset. The superior electrochemical and capacitive response revealed by the constituent electrodes can be ascribed to the synergistic effect. Direct growth of mesoporous PPy accross the FeO@CVO nanocane surface provides improved contact between the electroactive material and the current collector substrate.

Figure 11. (a) CV profiles of ASC at different scan rates. (Inset depicts an enlarged view of the CV profile @2 mV/s). (b) GCD profiles of ASC obtained at different current densities within an operating voltage window of 0─1.4 V. (c) Ragone plot of fabricated ASC. (d) Specific capacitance retention


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after 5000 GCD cycles (@1 A/g). (Inset illustrates a continuous GCD profile starting from 1st to 10th cycle at 1 A/g). 5. CONCLUSIONS This work demonstrates the concept of fabricating PPy/FeO@CVO hybrid electrode material for supercapacitor application through a simple, cost effective and environment friendly method that involves in-situ polymerization of pyrrole in the presence of hydrothermally synthesized iron oxide decorated cobalt vanadium oxide hydrate (FeO@CVO) nanocanes. Wrapping-up of mesoporous PPy over FeO@CVO nanocane surface results in a unique design that empowers the hybrid composite electrode material to achieve high specific capacity and cycle stability. A high specific capacitance of ~1202 F/g with excellent 96.5% cycle stability in PPy/FeO@CVO (@ 1 A/g) was obtained through judicious control of the amount of pyrrole monomer (0.3 ml, 96.7% (w/w)) during the polymerization. Additionally, the assembled ASC provides improved power and energy densities. All the reproducible experimental results suggest that this composite electrode material can be considered as a promising candidate for advance supercapacitor application. ASSOCIATED CONTENT Supporting Information. Average crystallite sizes of CVO, Elemental mapping and EDS pattern of CVO, FeO@CVO and PPy/FeO@CVO, morphology after 3000 charge-discharge cycles, atomic force microscopic studies, Raman and Fourier transformed infrared spectroscopic analysis, XPS spectra of CVO, Variation of specific capacitances with scan rates and current densities for the as-prepared electrode materials, CV of blank Ni foam, Electrochemical behavior of GNP as negative electrode. Notes: The authors declare no competing financial interest. ACKNOWLEDGEMENT We are very much thankful to Indian Institute of Technology Kharagpur for financial support.


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For Table of Contents Only

This study demonstrates the construction of a high performance robust mesoporous PPy wrapped FeO@CVO hybrid composite electrode with enhanced electrochemical performance as fabricated by following facile protocols. An asymmetric supercapacitor based on PPy/FeO@CVO//GNP has also been assembled that exhibits outstanding energy and power densities with long term cyclic stability.


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A Mesoporous High-Performance Supercapacitor ... - ACS Publications

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