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

Article pubs.acs.org/IECR

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* Materials Science Centre, Indian Institute of Technology Kharagpur, Kharagpur 721302, West Bengal, India S Supporting Information *

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 costeffective 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 a three-electrode configuration. Furthermore, a twoelectrode based asymmetric supercapacitor using PPy/FeO@ CVO as positive and graphene nanoplates (GNP) as negative electrodes and KOH-soaked paper as a separator reveals an outstanding energy density of 38.2 Wh/kg (power density 700 W/kg at 1 A/g) with amazing cyclic stability (95% capacitance retention after 5000 cycles), suggesting great prospective of the ASC for high-power device applications in modern electronic industries.

1. INTRODUCTION The universal requirement of a 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 a 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 composed of redox active materials, e.g., transition metal oxide, hydroxides, and conducting polymers that can store energy via a fast reversible redox reaction. From a materials point of view, activated carbon, carbon nanotubes, graphene, etc. are well-established materials for EDLC type electrodes,1 while various transition metal oxides and hydroxides such as 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 pyrophosphates,7 and ternary metal oxides, e.g., NiCo2O4,8 MnFe2O4,9 CoMoO4,10 etc., are recently utilized for highperformance electrode fabrication. Layered transition-metal © 2017 American Chemical Society

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 values along with superior cyclic 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 cyclic stability.16 He also investigated nickel vanadate and nickel oxide nanohybrid for excellent rate capability along with good cycle stability. Received: Revised: Accepted: Published: 2444

November 15, 2016 January 9, 2017 February 9, 2017 February 9, 2017 DOI: 10.1021/acs.iecr.6b04449 Ind. Eng. Chem. Res. 2017, 56, 2444−2457

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Industrial & Engineering Chemistry Research

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 S·cm−1.25 It has 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, thereby enhancing the overall performance with greater dimension stability. Furthermore, PPy has an intrinsic tendency to form network-like arrangements and simultaneously cross-links with each other in the presence of Fe3+ ions.27 Formation of a PPy network over FeO@CVO nanocanes simultaneously restricts the dimensional shrinkage and swelling during extreme cyclic 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 smaller. 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 a similar facile hydrothermal method. Finally, a robust, lightweight 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 the use of CVO (commonly used in batteries) as efficient supercapacitor electrode material through the 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 °C) in the 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 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 the 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 environmentally

The nanohybrid also shows a high power density accompanied by 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 to 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. 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 that has 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 toward 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 nanolevel 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 nanosheet 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 nanoarchitectures can be utilized for the 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), and poly(ethylenedioxythiophene) (PEDOT) can easily be prepared and employed as lightweight 2445

DOI: 10.1021/acs.iecr.6b04449 Ind. Eng. Chem. Res. 2017, 56, 2444−2457

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of ammonium persulfate dissolved in 20 mL of chilled deionized water was added to the above mixture dropwise under stirring condition 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 °C for 2 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 the presence of minute N-methyl-2-pyrrolidone (NMP) to form a paste. The prepared homogeneous 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 a Biologic SP-150 instrument with 1 M aqueous KOH 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.

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 cyclic conditions when assembled in a two-electrode configuration.

2. EXPERIMENTAL SECTION 2.1. Material Details. Cobalt(II) chloride hexahydrate (CoCl2·6H2O; molecular weight (MW) ≈ 237.93 g/mol), ammonium vanadate (NH4VO3; MW ≈ 116.98 g/mol), iron nitrate (Fe(NO3)3·9H2O; MW ≈ 404 g/mol), pyrrole monomer (MW ≈ 67.09 g/mol) were purchased from Merck. Potassium hydroxide (KOH), ammonium persulfate ((NH4)2S2O8; MW: 228.18 g/mol) were purchased from Loba Chemie Pvt. Ltd. Graphene nanoplates (Multilayer, carbon purity >99.5%; D = 5−25 μm; thickness: 8−10 nm) were purchased form J. K. Impex, Mumbai, India. All of the reagents used were of analytical grade purity and used without any further chemical purification. Deionized water, having a 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 of 1 M aqueous CoCl2 was mixed thoroughly with 20 mL of 1 M aqueous NH4VO3 in a glass beaker under 20 min of 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 that was then accumulated from the autoclave and washed with deionized water and ethanol 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 a similar hydrothermal procedure. In this process, 1 g of as-prepared CVO was mixed thoroughly with 30 mL of 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 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 weight percent 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 CoV2O6·2H 2O Nanocanes. Spherical PPy wrapped FeO@CVO hybrid composite material was obtained by a simple low-cost in-situ chemical oxidative polymerization of pyrrole monomer in the presence of FeO@CVO. At first, 0.3 mL of pyrrole monomer was sonicated with 10 mL of deionized water for 3 min to disperse it properly. In another beaker, 0.30 g of FeO@CVO powder was sonicated with 20 mL of 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

3. CHARACTERIZATIONS Powder X-ray diffraction (XRD) studies were performed using an X-Pert PRO diffractometer (PANalytical, Netherland), having monochromatic Cu Kα radiation (λ = 0.15418 nm) at a scan rate of 0.5°/min. Field emission scanning electron microscopic (FESEM) studies were performed by 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 transmission electron microscope (TEM). The surface topology of the electrode materials were detected by Nanonics Multiview 1000TM (Israel) SPM system with a quartz optical fiber tip (diameter of 20 nm, spring constant of 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 transform 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 of ∼1486 eV).30 Nitrogen adsorption isotherm was assessed at 77 K with a Quantachrome ChemBET TPR/TPD analyzer. The sample was degassed at 100 °C for 3 h under vacuum conditions before performing nitrogen adsorption kinetics. The effective exposed surface area was estimated by employing the Brunauer−Emmett−Teller (BET) model, and 2446


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Figure 1. Schematic representation for the fabrication of PPy/FeO@CVO composite electrode and PPy/FeO@CVO//GNP ASC device.

the pore size and volume was determined by the 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 electrodes individually, while platinum electrode (1 × 1 cm2) and saturated calomel electrode were used as the counter and reference electrodes, 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 a positive and GNP as a negative electrode, with KOH-soaked Whatman 40 filter paper operable within a 0−1.4 V potential window.

4. RESULTS AND DISCUSSION To explore the crystal structures of the as-prepared electrode materials, WAXD analysis was conducted by mounting samples on a transparent glass fiber plate, and they were placed in an X-ray diffractometer. Figure 2a−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 ≈ 17.6°, 22.02°, 22.4°, and 27.6° corresponds to (101), (102), (022), and (122) crystal planes, while other characteristic low-intensity peaks at 2θ ≈ 14°, 26.16°, 33.17°, 42.8°, and 52.9° stand for (020), (031), (201), (134), and (243) crystal planes, respectively (Figure 2a). The diffractogram of FeO@CVO

Figure 2. XRD patterns of (a) CVO, (b) FeO@CVO, (c) PPy/FeO@ CVO, and (d) pure PPy.

(as shown in Figure 2b) elaborates the formation of mixed iron oxides over the CVO nanocane surface. Along with most of the CVO signatures, the diffractogram exhibits mixed moderate- to low-intensity peaks at 2θ ≈ 13.6°, 31.5°, and 48.45° corresponds to (006), (113), and (127) plane of Fe2O3 (JCPDS no. 040-1139) While the signature at 2θ ≈ 30.04° indicates the presence of Fe3O4 (220) planes (JCPDS no. 01-089-0950). 2447


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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.

Furthermore, diffraction peaks at 2θ ≈ 29° and 50.75° stands for Fe−OOH (003) and (005) planes (JCPDS no. 00-0461315), respectively. This result well depicts the formation of mixed iron oxide (solely Fe2O3 with a small amount of Fe3O4 and Fe−OOH) over the 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 ≈ 25−28° (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 a less-intense broad PPy (104) hump. It has been observed that the signature of PPy has been suppressed in the appearance of highly intense FeO@CVO peaks. The obtained result suggests an amorphous PPy network successfully wrapped over FeO@CVO nanocane arrays. The XRD pattern of pure PPy is referenced in Figure 2d.31 Probable intermolecular interaction between PPy moiety and FeO@ CVO, as proposed in Figure 1, may be the possible reason behind a marginal shifting of the PPy peak position in PPy/ FeO@CVO in contrast to that in pure PPy. The probable 3D crystal structure of CVO (as shown in Figure S1 in SI) 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 the Scherrer equation (eq 1) for the four most-intense peaks appearing at various positions with a 2θ value of 17.6°, 22.02°, 22.4°, and 27.6° (as tabulated in Table S1 in SI), which is further supported by high-resolution transmission electron microscopic studies. CVO dhkl =

0.9 × λ 180° × β × cos θ π

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 the PPy/FeO@CVO nanocomposite as represented in Figure 3e,f illustrates that all of the entangled nanocane arrays were well-wrapped with granulartype PPy network with average diameter in nanometric scale length (Figure 3e). However, some non-uniformity in the coating was observed due to a smaller 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 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 the 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 the electron-deficient H atom directly attached to N atom in a PPy moiety and the 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. The elemental mapping of the as-prepared electrode materials, as shown in Figure S2a−c in SI, depicts a homogeneous distribution of the constituent elements. The presence of Co, V, O (Figure S2a), a small amount of Fe along with CVO (Figure S2b), and C and N of PPy (Figure S2c) can be corroborated. An energy-dispersive X-ray line pattern (Figure S3 in SI) also demonstrates the similar information.

(1)

where λ denotes the wavelength of Cu Kα, β is the full width at half-maximum, and θ is the Bragg angle. The morphological characteristics were ascertained through FESEM studies and represented in Figure 3a−h. The FESEM images depicted in Figure 3a,b exhibit grass-like morphology of CoV2O6·2H2O nanocanes. 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 2448


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Figure 4. TEM images of (a, b) CVO, (d, e) FeO@CVO, (g, h) PPy/FeO@CVO, and (i) pure PPy. Panels c and f represent the SAED patterns of CVO and FeO@CVO, respectively.

prepared without using FeO@CVO. PPy shows similar granular and agglomerated morphology, which affirms the FESEM results. Raman spectroscopy (as elaborated in Figure S6 under S3 in SI) and FTIR spectroscopic analysis (Figure S7 under S4 in SI) were carried out to determine the molecular structure, interactions and bond information on the as-prepared electrode materials. 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 5b−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. The Gaussian fitting method has been utilized for all the depth profile scan plots of FeO@CVO (Figure 5b−e) 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 of 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 the Gaussian fitting method, four peaks have been

The morphological changes obtained after 3000 consecutive charge−discharge cycles were also investigated (Figure S4 under S1 in SI). High-resolution transmission electron microscopic (HRTEM) and atomic force microscopic analysis (as shown in Figure S5 under S2 in SI) also support the features obtained from FESEM studies. Figure 4a−i represents the highresolution 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 surfaces. The nanocanes assembled together in a random array (Figure 4a), and its diameter varies between 55−65 nm (Figure 4b).32 Selected area electron diffraction patterns (SAED) as illustrated in Figure 4c expose the polycrystalline nature of CVO. More specifically, the grain boundaries are present in much larger portions than the single crystal, which conveys the easy transport of electrolyte ions.17 Figure 4d,e shows the TEM images of FeO@CVO nanohybrid, in which 5−10 nm particles of spherical iron oxides were found to decorate over the surfaces of nanocanes. The SAED pattern 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,h depicts the obtained low- and high-magnification TEM images of the PPy/FeO@CVO nanocomposite, accordingly. The presence of PPy can be easily determined from both of 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 the neighboring PPy chains. Figure 4i indicates the TEM image of pure PPy 2449


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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.

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 in SI) 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. 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. BET analysis was executed for PPy/FeO@CVO hybrid electrode material to determine the specific surface area and the

identified from the V 2p spectrum (Figure 5c). V 2p3/2 and 2p1/2 peaks appear at a binding energy of 516.7 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 the 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, consistent 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 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 above mentioned peaks for Fe 2p appeared in FeO@CVO support the presence of iron in both 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 and 712.3 eV, respectively. The satellite peak appears at 717 eV describes the presence of

Table 1. XPS−EDX analysis of CVO and FeO@CVO elements present (atomic percentage)

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materials

Co

V

Fe

O

CVO (Figure S8 in SI) FeO@CVO

15.40 8.61

23.43 25.92

0.00 6.73

61.16 58.74


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Figure 6. N2 sorption isotherm (a) and pore-size distribution (b) of PPy/FeO@CVO nanocomposite measured at 77 K.

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

pore-size distributions. Figure 6a represents the nitrogen adsorption−desorption isotherm of PPy/FeO@CVO at a temperature of 77 K. The isotherm follows a Type-III pattern, having an 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. The 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, a 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 the BJH method. 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 of 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 little or no variation. 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 7a−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 an increase of the scan rate from 2 to 100 mV/s, the anodic peak shifts toward higher

Co2 + + 3OH− = CoOOH + H 2O + e−

(I)

CoOOH + OH− = CoO2 + H 2O + e−

(II)

In an alkaline medium, equilibrium exists between the 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 FESEM and TEM). The plausible redox reaction mechanism for FeO@CVO can be described as follows: Fe3 + + Co2 + = Fe 2 + + 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 an increasing 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 among the other two, indicating a high specific capacitance value. Figure 7d demonstrates the CV plots of pure PPy, in which 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 small compared to all of 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 a 2 mV/s scan rate for easy assessment. The current response of PPy/FeO@CVO (III) is greater than both FeO@CVO (II) and CVO (I), suggesting PPy/FeO@CVO as potential candidate for efficient electrode materials. The inset of Figure 7e illustrates the enlarged view of 2451


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Figure 7. Cyclic voltammetry plots of (a) CVO, (b) FeO@CVO, (c) PPy/FeO@CVO, and (d) pure PPy at different scan rates and (e) CV plot altogether at a 2 mV/s scan rate (I, CVO; II, FeO@CVO; III, PPy/FeO@CVO; and IV, PPy).

Table 2. Values of Specific Capacitance with Scan Rates for All of the As-Prepared Materials specific capacitance (F/g) at different scan rates materials

2 mV/s

5 mV/s

10 mV/s

30 mV/s

50 mV/s

100 mV/S

CVO (mass: 0.0135 g) FeO@CVO (mass: 0.0123 g) PPy/FeO@CVO (mass: 0.0116 g) PPy (pure) (mass: 0.0109 g)

627 931 1109 324

542 842 1004 260

465 721 887 213

334 601 762 153

266 522 655 106

172 369 500 71

the preparation of the nanocomposite. Beyond this optimum amount of pyrrole, the amount of PPy increases, leading to the complete coating of FeO@CVO nanocanes rather than wrapping by PPy. In that circumstance, electrolyte will not be able to penetrate in the CVO phase, and ion transportation will be prohibited. Hence, capacitance will be decreased with an increase in the amount of pyrrole. 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 eq 3, as follows:10,23

all of the CV curves obtained at the lowest scan rate for better clarity. The specific capacitance of all as-prepared electrode materials were calculated from their respective CV plots using eq 2, as described below:10,23 V

specific capacitance (Cs) =

∫V 2 i(V )dv 1

(V2 − V1)vm

(2)

where V1 and V2 represent the lower and the upper potential limits, i indicates the current response, v 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 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 in SI). 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 the presence of conducting PPy. As mentioned earlier, 0.3 mL of pyrrole monomer was used as the optimum amount during

specific capacitance (Cs) =

i × Δt m × Δv

(3)

Here, Cs is the calculated specific capacitances in F/g, (i/m) represents the current density in A/g, and 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 8a−d. All of the obtained charge−discharge curves show nonlinear behavior that clearly elaborates the pseudocapacitive nature of the as-prepared materials under the applied potential window. Figure 8a,b depicts the GCD curves of CVO and FeO@CVO at different current densities, respectively. The specific capacitance 2452


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Figure 8. GCD plots of (a) CVO, (b) FeO@CVO, (c) PPy/FeO@CVO, and (d) pure PPy at different current densities and (e) altogether at a 1 A/g current density.

electrolyte ions within the pores of the electrode materials and the lack of availability of the redox sites at high current densities. Furthermore, a brief comparison of our acquired results with other nanostructured morphologies reported elsewhere has been tabulated in Table S2 in SI. 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

values obtained for CVO and FeO@CVO were 708 and 968 F/g, respectively, at 1 A/g current density. The GCD plots of PPy/FeO@CVO (as shown in Figure 8c) display the 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 for enhancing the electrochemical performance of the as-prepared hybrid composite. Moreover, PPy/FeO@CVO, being more electrically conductive, shows the smallest IR drop during discharging among all the others. Presence of mesoporous PPy also increases the overall effective surface area for convenient electrode−electrolyte interaction, thereby facilitating the redox process during the 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 8e represents GCD curves of all as-prepared electrode materials collectively at 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 in SI). This is perhaps due to lesser diffusion of the Table 3. Values of Specific Capacitances with Current Densities for the Electrode Materials specific capacitance (F/g) at different current densities materials

1 A/g

1.5 A/g

2 A/g

3 A/g

CVO (mass: 0.0135 g) FeO@CVO (mass: 0.0123 g) PPy/FeO@CVO (mass: 0.0116 g) PPy (pure) (mass: 0.0109 g)

708 968 1202 258

510 642 831 189

424 440 556 76

318 360 456 3.6

Figure 9. Cyclic stability plots of CVO, FeO@CVO, PPy/FeO@CVO, and pure PPy at 1 A/g. 2453


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electrode materials were calculated to be 1.24, 0.73, 0.51 Ω for CVO, FeO@CVO, and 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 a greater exposure of PPy/FeO@CVO hybrid nanocomposite toward a 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 being prone toward 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 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 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 provides 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 (Figure S11 under S5 in SI). In accordance with the acquired results, an asymmetric supercapacitor (ASC) based on two-electrode configuration has been fabricated by employing PPy/FeO@CVO as positive and GNP as negative electrodes in the presence of KOH-soaked Whatman 40 filter paper as a separator. The porous separator soaked with electrolyte solution restricts the contact between the two electrodes, while it 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 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 and discharged at 2 A/g for ∼100 repetitive cycles 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 eq 4 as follows:

FeO@CVO (Figure 1) and stability of the 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 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) versus real component of impedance (Zreal)) were exhibited in Figure 10. The inset of

Figure 10. Nyquist plots of CVO, FeO@CVO, and PPy/FeO@CVO. Impedance plots at a 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.

Figure 10 depicts the impedance behavior of the electrode materials at a 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 all of 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 a similar kind of behavior with a starting semicircular zone at a 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 a high frequency region determines solution resistance (Rs) of the materials. The diffusion phenomenon, i.e., Warburg behavior of the electrode materials can be determined from the slope of the steeper linear part at a 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 capacitor.23 The solution resistances (Rs) of all as-prepared

q = C × ΔE × m

(4)

where C, ΔE, and m are the specific capacitance (F/g), the working potential range (V), and the mass of the active electrode material (g), respectively. The necessary mass balancing 2454


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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 after 5000 GCD cycles (@1 A/g). (Inset illustrates a continuous GCD profile starting from 1st to 10th cycle at 1 A/g).

equation for the fabrication of an ASC can be expressed as follows:41 m+ C × ΔE− = − m− C+ × ΔE+

EASC =

1 × CASC × (ΔV )2 2

PASC = EASC /T (5)

(6) (7)

Here, EASC denotes the energy density in Wh/kg, ΔV stands for the voltage drop during discharge (V), CASC is the total specific capacitance of the ASC (F/g), PASC signifies power density in W/kg, and T represents the discharge time. Figure 11c illustrates the Ragone plot of the fabricated ASC based on a 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 and CNT-based symmetric supercapcitors reported elsewhere, e.g., CNT//CNT symmetric supercapacitor (