Advanced asymmetric supercapacitor with NiCo2O4 nanoparticles and nanowires electrodes: A comparative morphological hierarchy

Journal of Alloys and Compounds 821 (2020) 153503

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Advanced asymmetric supercapacitor with NiCo2O4 nanoparticles and nanowires electrodes: A comparative morphological hierarchy Mahasweta Chatterjee a, Samik Saha b, Sachindranath Das b, Swapan Kumar Pradhan a, * a b

Department of Physics, The University of Burdwan, Burdwan-713104, West Bengal, India Department of Instrumental Science, Jadavpur University, Kolkata-700032, India

a r t i c l e i n f o

a b s t r a c t

Article history: Received 16 October 2019 Received in revised form 20 December 2019 Accepted 21 December 2019 Available online 24 December 2019

In the present work, hydrothermal and wet chemical methods are adopted to fabricate NiCo2O4 nanowires (NiCo-NW) and NiCo2O4 nanoparticles (NiCo-NP) respectively. Owing to the mesoporous nature of these subunits, fast and convenient electron-ion transport and redox reaction, NiCo-NW achieves excellent electrochemical performance. Structure and microstructural characterizations of these samples are carried out by analyzing X-ray diffraction data employing the Rietveld method of structure refinement method and analyzing HRTEM, FESEM images and FTIR spectra. The low dimensional NiCo-NP is found to provide superior electrochemical performance than the NiCo-NW (~13 nm) due to its smaller particle size (~9 nm). This porous structure effectively helps in better transport of ions in the electrolyte. It manifests high specific capacitance 1066.03 F g
1 and enormous areal capacitance up to 5.96 F cm
2 whereas NiCo-NW exhibits specific capacitance up to 880.72 F g
1 and high areal capacitance of 4.93 F cm
2. An asymmetric supercapacitor (ASC) has been fabricated with NiCo-NP and activated carbon as positive and negative electrodes respectively in 1 M Na2SO4 electrolyte medium. This device offers maximum specific energy 59.56 Wh Kg
1 and maximum power density 3403 W kg
1 with a high energy density of 4.197 Wh Kg
1 and shows excellent cyclic stability. © 2019 Published by Elsevier B.V.

Keywords: NiCo2O4 3D nanowires Porous structure Asymmetric supercapacitor device Energy storage

1. Introduction In recent years, enormous attention has been drawn to develop novel materials and devices for the new renewable and sustainable energy sources with high efficiency, high reliability and high energy density. The supercapacitor has been used massively in last few decades as a green energy storage device combining the features of the conventional capacitor (high power density, long cycling life) and rechargeable batteries (high energy density) [1e8]. Based on the charge storage mechanism supercapacitors are of two types: (i) electric double-layer capacitor(EDLC), and (ii) pseudocapacitors. For EDLCs electric energy is stored by separation of charge in Helmholtz double-layer and for pseudocapacitor storage of electric energy is achieved by a faradaic redox reaction with charge transfer [8e10]. Various carbonaceous materials like activated carbon, CNT, graphene are being used as electrode materials for EDLCs for their higher surface area with a porous surface and electrically intercalated networks. EDLCs show high power density, better cycle life

* Corresponding author. E-mail address: [email protected] (S.K. Pradhan). https://doi.org/10.1016/j.jallcom.2019.153503 0925-8388/© 2019 Published by Elsevier B.V.

than pseudocapacitor but possess very low specific capacitance. However, due to fast multi electro-redox reaction, pseudocapacitors possess higher specific capacitance, higher energy density than observed in EDLCs [11,12], but it leads to deficient cycle stability because of redox reaction like a battery. The primary focus of the present work is to improve cell voltage and energy density by developing an ASC device in which (EDLC) electrode has been used as the negative electrode and redox-active transition metal oxides as a positive electrode. The maximum operating voltage in the cell system can be reached by using different potential windows of the two-electrode system. Primarily, activated carbon has been used as the negative electrode and transition metal oxides as a positive electrode. So, the main focus of ASC is to develop better metal oxides for advanced positive electrode [3,13]. Various metal oxides and hydroxides with their variable valence states had been widely used for electrode materials in pseudocapacitors [14,15]. Attempts had been made to prepare inexpensive metal oxides like Co3O4 [16,17], NiO [10,18], MnO2 [19], V2O5 [20], Fe2O3 [21] for high theoretical capacitance and low toxicity. Both Ni and Co-based materials were considered to be the most admirable

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candidate for pseudocapacitor application for their high redox activity. There is a various advantage of ternary oxides such as Ni-Mn [22,23], Ni-Co [1e10], Co-Mn [24], Zn-Co [25], in comparison to the individual metal oxides (such as NiO, Co3O4, Fe2O3). Ternary oxides or composite oxides encourage electrochemical performance due to the co-operative effect of pure ones. The ternary system possesses surface defects and provides large electrode-electrolyte contact surface area which enhances with the redox reaction. Among various spinel materials, NiCo2O4 had been used extensively as a promising material because of its multiple valence states, various controllable morphologies, superior electronic conductivity, high capability, non-toxicity, better electrochemical property than the cobalt and nickel oxides [11e15]. Many researchers worked on NiCo2O4 and demonstrated it as an advisable material for pseudocapacitor application because of contributions from nickel and cobalt ions of different valence states [1e13]. In NiCo2O4, rate capability and kinetics of diffusions were improved due to the short transport pathways of electrons and ions [7]. The performance of electrochemical measurement can be revamped by synthesizing proper nanostructure with various morphologies such as nanowires [26,27], nanoneedles [28], nanosheets [1,29], nanoflowers [4,30], nanoflakes [2], nanoparticles [31] by hydrothermal, sol-gel, wet chemical, solvothermal or any other methods. For this distinct various structural modifications, this spinel nickel cobaltite has been used in various fields such as in lithium-ion batteries [32], electrocatalysis [33], magnetic materials [34], photodetector [35], electrical transport [36] and mostly in electrochemical applications [1e10]. The prime intention of the present work is to design metal oxide nanostructures with proper controllable morphology, size, excellent specific surface area with better electrical conductivity for energy storage devices. A facile hydrothermal template free strategy and a simple wet chemical technique have been adapted to prepare mesoporous NiCo2O4 nanowire array (NiCo-NW) and porous NiCo2O4 nanoparticles (NiCo-NP) respectively. The nickel cobaltites synthesized by the above two methods produce porous structure during calcination when precursors decomposed with a release of a huge amount of gas and create a large number of pores in sintered samples. The difference in electrochemical performance with nanowires and nanoparticles has been explored in the present study. It is noticed that nickel cobaltite nanoparticles manifest exceptional areal capacitance than nickel cobaltite nanowire array. The dependency of supercapacitive behaviour on the size and shape of the nanoparticles has been illustrated in details. 2. Experimental 2.1. Precursor materials All the chemicals (Nickel nitrate [Ni(NO3)2$6H2O] (Loba chemicals). Cobalt Nitrate [Co(NO)3$6H2O,97%], Urea [CH4N2O, 99.5%] (Merck chemicals), Ammonia [NH3]) are of analytical grade and have been used in the experiment without further purification. 2.2. Preparation of NiCo2O4 nanoparticles and nanowires 2.2.1. Wet chemical method A 5 mM cobalt nitrate and 10 mM nickel nitrate were dissolved in 100 mL deionized water and mixed well with each other. After stirring for 30min, 20 mM urea was added with it as a precipitating agent. The mixture was then stirred for another 4h to obtain a complete homogeneous solution. During this stirring process, the pH of the solution was maintained at 9 by slowly adding a small amount of ammonia into the solution. The resulting solution

was filtered and washed several times by deionized water and ethanol. The collected precipitate was then dried at 60  C and finally annealed at 300  C for 8h. This sample was labelled as NiCo-NP. 2.2.2. Hydrothermal method A 3 mM nickel nitrate and 6 mM cobalt nitrate were dissolved in 75 mL of deionized water. The mixture was stirred for 1h. Then 40 mM urea was added to the mixture and the solution was kept stirring for another 3h. The pH of the solution was maintained at 10 by adding ammonia into it. The mixed solution was then sealed in Teflon lined stainless steel autoclave and maintained at 120  C for 6h. Then the resulting precursor was allowed to cool in room temperature in a normal cooling process. The final precursor was then collected from autoclave washed with ethanol and distilled water several times and the precipitates were collected after filtration. Filtered to get cobalt-nickel hydroxide. The precipitate was then dried at 60  C for 8h and finally, the dried precipitate was annealed at 350  C for 2h. This sample is named as NiCo-NW. 2.3. Material characterization The structure and microstructure of the materials were explored by recording X-ray diffraction data utilizing Bruker D8 Advanced diffractometer (Ni-filtered CuKa radiation generated at 40 KV and 40 mA). The data were recorded within the angular range 2q ¼ 20 e80 in step scan mode 0.02 2q and time/step 5e10 s which is suitable for Rietveld refinement method. Rietveld software Maud version 2.7 was used to perform simultaneous refinement of both structure and microstructure parameters using Marquardt least-square refinement method. The surface morphology of the samples was characterized by scanning electron microscope (FESEM, JEOL, Model: JSM-6390LV). Details microstructure and morphological analysis were carried out by analyzing images obtained from a transmission electron microscope (TEM) (JEOL, JEM 1400 plus) operated at 120 kV. For further structural confirmation by spectroscopy analysis, infrared (FT-IR) spectrometer (PERKIN ELMER, FRONTIER) was used in transmission mode over a range 400e4000 cm
1 using a KBr standard. 2.4. Electrode fabrication and electrochemical measurements for supercapacitor The working electrode materials were prepared by mixing synthesized sample, acetylene black, polyvinylidene fluoride (PVDF) in a ratio of 85:10:5. Then this hybrid was added to the Nmethyl pyrrolidine (NMP) and kept in sonication for 30min. A geltype solution was formed and it was drop-casted on one end of Teflon coated graphite rod and dried for 8h at 60  C. The electrochemical performance of these two materials was carried out as a supercapacitor electrode in a three-electrode system containing 1 M Na2SO4 aqueous solution as the electrolyte with Pt-foil and Ag/ AgCl electrode used as counter electrode and reference electrode respectively. For the Ag/AgCl reference electrode, saturated KCl was used with 4 M concentration. The electrochemical performance of the Asymmetric Supercapacitor (ASC) device was accomplished in a two-electrode cell containing 1 M Na2SO4 as an electrolyte solution. The anodes of ASC was assembled by depositing synthesized materials on stainless steel sheet (2 mg cm
2) and activated carbon was deposited on stainless steel for use as negative electrodes (cathodes). The negative electrode was adopted by 90 wt% activated carbon (AC) and 10 wt% PVDF binder dispersed in NMP solution to produce a homogeneous paste. Then this paste was deposited on an area of

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6 cm2 (2 cm  3 cm). The prepared electrodes were dried at 330 K for 5 h. All the electrochemical measurements were done at 300 K. The three main electrochemical properties namely, cyclic voltammetry (CV), galvanostatic charge-discharge (GCD) and electrochemical impedance spectroscopy (EIS) were executed with the help of CS313 (Corrtest, China) multi-channel electrochemical analyzer. 2.5. Method of analysis In the present study, Rietveld’s power structure refinement method [37e42] has been adopted to determine several structural parameters such as atomic coordinates, occupancies, lattice parameters, thermal parameters and micro/nanostructural parameters such as particle size and r.m.s. lattice strain of NiCo2O4 compounds synthesized by different chemical methods. The Rietveld software MAUD [41] has been utilized to refine both the structural and microstructure parameters simultaneously. The refinement process endows with a least-square refinement procedure (Marquardt least-square) to refine different parameters. By adopting Marquardt least-squares procedure the difference between observed (Io) and simulated (Ic) intensities of X-ray powder diffraction pattern is minimized and minimization is monitored using the reliability index parameter Rwp (weighted residual error), and Rexp (expected error). This brings out the value of goodness of fitting(GOF) [38e40],

GOF ¼

Rwp Rexp

The refinement through successive iterative method continues until the GOF value approaches towards unity. A pseudo-Voigt (pV) profile fitting function is used to fit XRD pattern which accounts for the individual contribution of small crystallite size and lattice strain to the instrumental error-corrected true peak broadening crystallite. 3. Results and discussion

3

also support the statement i.e crystal size of NiCo-NW is greater than NiCo-NP. 3.2. Morphology and growth mechanism revealed by FESEM and TEM images analyses Fig. 2 depicts the FESEM images of as-prepared 3D mesoporous nanowires (Figs.aed) and 1D porous nanoparticles (Figs.eef) synthesized by hydrothermal and wet chemical routs respectively. From the FESEM images (a-d) it is clear that NiCo-NW porous nanowires consist of numerous interconnected nanoparticle subunits with individual size of 14e16 nm (inset of Fig. 2(c)). The diameter of the individual subunits is compatible with the result obtained from Rietveld analysis. The magnified image of NiCo-NW is shown in Fig. 2(c). Fig. 2(b) manifests the 3D structure of nickel cobaltite nanowire array with uniform diameter whose tips are vertically aligned, some of them are interconnected and some are separated. Tips of vertically aligned nanowires (3D) appear (Fig. 2(c)) as an assembly of mesoporous spherical nanoparticles of 70e80 nm. However, the magnified image of such a spherical particle clearly reveals the internal microstructure of the tips, which primarily consist of a large number of smaller nanoparticle subunits of ~13 nm diameter (Fig. 2(d)). Fig. 2(eef) reveals a 1D porous structure of nickel cobaltite nanoparticles with an average diameter of 8e10 nm synthesized by wet chemical route. It is evident from these images that these spherical nanoparticles are almost uniform in size and shape and the compound is significantly porous in nature. Fig. 3(aeb, c-d) elucidates the TEM images of NiCo2O4 nanowires and nanoparticles. From the TEM image (Fig. 3(a)) it is evident that these 3D nanowires are built up with the agglomeration of numerous interconnected nanoparticles of uniform size and shape. It can also is seen that various numerous nanoparticles agglomerate together to form these 3D nanowires. This compound possesses a unique nanowire array with the highly porous structure on a large scale. The NiCo2O4 nanowires have an average length of 200e300 nm with average diameter 70e80 nm. On the other hand,

3.1. Phase identification of nanocomposites by analyzing XRD data Fig. 1(a) and (b) illustrate the XRD patterns of the as-synthesized NiCo2O4 compounds by the hydrothermal route and wet chemical route respectively. Sharp reflections indicate that both samples are well crystalline in nature and patterns are almost identical. XRD patterns are identified as belonging to the cubic spinel structure and have been indexed accordingly (COD no.9001364; Sp. Gr. Fd3m; a ¼ 8.0126 Å.) It is evident from the patterns that intensity ratio I311: I400 for wet chemical route sample is lower than that of the hydrothermal route. The absence of any secondary/impurity peak in the XRD patterns confirms the phase purity of the compounds. In general, chemically synthesized the nickel-cobaltite exhibits in mixedspinel structure in which eight tetrahedral sites are occupied by six Co3þ and two Ni2þ ions, sixteen octahedral sites are occupied by six Ni2þ and ten Co3þ ions. However, in Fig. 1(a) the XRD pattern is fitted well in accordance with the normal spinel structure i.e. considering tetrahedral sites are occupied with Ni2þ ions and octahedral sites are occupied with Co3þ ions and that of Fig. 1(b) with mixed spinel structure (see Table 1). A close observation of the XRD pattern also discloses that the intensity ratio of (311) reflection to (400) reflection is stronger in Fig. 1(a) than that of Fig. 1(b). The difference in intensity distribution originates from the difference in cationic distributions of normal and mixed spinel structure. Results from Rietveld analysis

Fig. 1. Typical Rietveld analysis output of the XRD patterns of NiCo2O4 samples synthesized by (a) hydrothermal route (b) wet chemical route. Experimental (IO) data are shown as red hollow spheres (o), simulated patterns (IC) are shown by blue solid lines, (IOeIC) (lower line) represents the corresponding difference between observed and calculated pattern. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)

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Table 1 Values of different microstructure parameters obtained from Rietveld refinement. Sample

NiCoNW

Morpho-logy

3D nanowire array

NiCo-NP Spherical porous nanoparticles

Crystal system

Cell length (Å) a ¼ b ¼ c Crystal-lite size (nm) R.m.s. lattice strain(10
4)

Cationic distribution

Cubic Sp.Gr. Fd3m

8.126

13.83

1.988

Ni2þ 8 Co3þ0

Cubic Sp.Gr. Fd3m

8.149

9.20

5.150

Co3þ6

GoF

Tetra hedral site Octa- hedral site

Ni2þ 2

Ni2þ 0 Co3þ 16 Co3þ 10 Ni2þ 6

1.07

1.26

Fig. 2. FESEM images of (aed) NiCo-NW and (eef) NiCo-NP samples.

NiCo-NP (Fig. 3(c)) exhibits a porous structure composed of nanoparticles with very small spherical particles with diameter ~9e10 nm, consistent with the FESEM and Rietveld results. Both these compounds suppose to become mesoporous/porous due to the release of gaseous elements during the long term thermal decompositions of precursors in the course of chemical reactions. The selected area diffraction pattern (SAED) of both nanowires and nanoparticles are represented in Fig. 3(b) and (d) respectively. Evidently, ring patterns of nanowires and nanoparticle are almost identical and they are indexed with the cubic NiCo2O4 spinel structures. Moreover, both of them show well-defined rings which indicate the polycrystalline characteristics of these compounds. In

particular, 1D porous nanoparticle structure exposes high specific surface area than that of 3D nanowire array due to the smaller particle size. So, it is expected that nanoparticle with porous microstructure exhibits excellent electrochemical results than nanowire array when evaluated as salient low-cost electrode materials for future generation electrochemical supercapacitor. Due to the better pore volume of NiCo-NP with uniform shape and size (Fig. 3(c)), it provides accessible pathways for fast penetration of electrolyte and can access better redox reaction [4,9]. 3.3. FTIR spectra characterization FTIR spectra of NiCo-NW and NiCo-NP composites are clearly

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5

Fig. 3. (a) TEM image, (b) SAED pattern of NiCo-NW. and (c) TEM image, (d) SAED pattern of NiCo-NP samples.

vibration mode of H2O [44]. The strong peaks at 657.39 cm
1 and 666.71 cm
1 correspond to octahedral coordinated Co3þ-O bond and the peaks at 568.5 and 568.7 cm
1 refer to the stretching vibration mode of tetrahedrally coordinated Ni2þ-O bond respectively [45]. These two peaks are the characteristics peaks of spinel materials. 3.4. Electrochemical characterizations of NiCo-NP and NiCo-NW compounds

Fig. 4. FTIR spectra of NiCo-NP and NiCo-NW compounds.

shown in Fig. 4. The band located at 3402.1 cm
1 of NiCo-NW is attributed to OeH vibration band in hydroxyl groups, which may arise due to H2O absorption on the surface of nanowires [43]. Bands situated at 1620.3 cm
1 and 1632.6 cm
1 are due to bending

To study the applicability of nickel-cobalt binary oxide in an energy storage device, the electrochemical performance of these two materials have been verified and compared with their respective cyclic voltammetry (CV) and galvanostatic chargedischarge (GCD) performances. Fig. 5(a) and (b) show the CV curve of NiCo-NP and NiCo-NW respectively at a scanning rate of 2e100 mVs
1. Both NiCo-NP and NiCo-NW compounds exhibit wide potential windows 1.68 V and 1.48 V respectively in the presence of Na salt electrolyte. Fig. 5(c) shows the CV curves of these two materials at a scan rate of 100 mVs
1. It is evident that the shape and size of the CV curve of NiCo-NP and NiCo-NW compounds are almost equal. Apparently, however, the area of the CV curve of NiCo-NP electrode is greater than that of NiCo-NW, which agrees well with the calculative value. It can be elucidated as

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Fig. 5. CV curves of the (a) NiCo-NP and (b) NiCo-NW electrodes at different scan rates ranging from 2 to 100 mV s
1, (c) comparison of the CV curve of NiCo-NW and NiCo-NP at a scan rate of 100 mV s
1 and (d) corresponding specific capacitance as a function of scan rate.

NiCo-NP exhibits a more porous structure than NiCo-NW and the surface area of NiCo-NP is greater than that of NiCo-NW as the particle size of NiCo-NP is less than that of NiCo-NW. Eventually, the porous structure of NiCo-NP confirms the abundant surface defects and good conductivity which helps the ion to penetrate easily in the electrode materials [1,3,46]. Moreover, though these compounds belong to cubic spinel, the cationic distributions (Ni2þ, Co2þ, Co3þ) among the eight tetrahedral and sixteen octahedral positions are different as they are synthesized by two different routes. All these parameters have played crucial roles in describing different potential windows for these compounds with the same chemical compositions but growth with different morphologies. For NiCo-NW compound there is one oxidation peak (at 1.1 V) observed the anodic process at a scan rate of 20 mVs
1 which indicates that the capacitive characteristics are governed mainly by Faradaic reaction [47]. The anodic peak shifts towards the negative potential as the scan rate increases due to the limited diffusion time. In CV, the energy storage mechanism originating from surface redox reaction in neutral Na2SO4 electrolyte proceeds based on the following equation [2e7]: NiCo2O4 þ Naþ þ e
4 NaNiCo2O4

(1)

the reaction takes place due to surface adsorption and desorption of Naþ ion. Na ions enhance supercapacitive activity of NiCo2O4 by acting as charge compensation agent.

The specific capacitance values of these materials have been calculated at different scan rates from the CV curves by the equation,

Cm ¼

i 2mv

(2)

where m and v are the mass of the electrode materials and scan rate respectively. The current (i) can be illustrated by integrating the area of the CV curve, which is described by, VðC

i ¼ Va

iðVÞdV Vc
Va

(3)

where Va and Vc are the lowest and highest voltage of the potential range [2e10,47e50]. The area of the CV curve decreases with the decrease of scan rate. The maximum specific capacitance of NiCoNP and NiCo-NW compounds are 1066.03F g
1 and 880.72 F g
1 respectively obtained at a scan rate of 2 mVs
1. The decreasing rate of specific capacitance of both the materials with scan rate is shown in Fig. 5(d). Interestingly, at the beginning of the profiles, the NiCo-NP electrode demonstrates a higher specific capacitance than that of NiCo-NW, yet its specific capacity declines sharply with the increase of scan rates than that of NiCo-NW compound. It is

M. Chatterjee et al. / Journal of Alloys and Compounds 821 (2020) 153503

suspected that slowing down of scan rate allow electrolyte to penetrate into the pores easily and get contact with the greater active surface area of the electrode materials for a long time and as a result, more charge is stored on the surface of a electrode and yields a higher capacitance [49e52]. At higher scan rates due to the slow diffusion rate of redox electrolyte and higher mobility, there is less chance of penetrating the electrolyte into the inner surface of electrode materials, so electrolyte comes into contact only in the outer surface of the electrode and consequently low capacitance is observed. The total amount of charge stored in electrode materials is the sum of both Faradaic current and non- Faradaic capacitive current. It indicates that there may be two effects: Faradaic effect from charge transfer contribution and non-faradaic double layer effect. According to Power Law, the CV current can be related to scanning rate by Refs. [52e54], i ¼ avs

(4)

where ‘a’ and ‘s’ are the adjustable parameter, v is the scan rate and ‘s’ value can be calculated from the slope of (logip) vs (log v) plot at a fixed potential V. From the value of ‘s’ behaviour of electrode material can be determined. When s ¼ 0.5, the charge storage mechanism is mainly Faradaic i.e diffusion-controlled type and at s ¼ 1, it becomes fully capacitive i.e., non-Faradaic or non-diffusion controlled process [52e55]. Fig. 6(a) shows the log ip vs log v plot for NiCo-NP and NiCo-NW electrode materials. From the slope best linear fit data obtained values of s are 0.773 and 0.889 for NiCo-NP and NiCo-NW respectively. The value of s interprets that storage mechanism involves both capacitive and Faradaic processes. For further manifestation, the peak current value can be expressed by the combined effect of capacitive current and adsorption current as [56e58],

7

i(V) ¼ k1 (n) þ k2 (n1/2)

(5)

i(V)/n1/2 ¼ k1n1/2 þ k2

(6)

where k1 (n) and k2 (n1/2) are the contributions of capacitive and adsorption currents. Values of k1 and k2 are determined from slope and intercepts of the linear fit of i(V)/n1/2 vs. n1/2 plot which are 1.793  10
4 and 4.92710  10
4 for NiCo-NP (Fig. 6b) and 2.109  10
4 and 2.472  10
4 for NiCo-NW (Fig. 6(b)) respectively. From the Trassati’s equation (Eq. (7)) the total specific capacitance can be expressed in terms of contribution of two capacitances; one is from the inner surface and other is from the outer surface of the electrode material [59] as, Ctotal ¼ Cin þCout (F g
1)

(7)

where Cin is the capacitance due to the inner surface effect i.e due to the Faradaic redox reaction and Cout is the capacitance due to the outer surface effect i.e due to the capacitive current. Fig. 6(c) represents the linear plot of specific capacitance vs n1/2 of NiCo-NP and NiCo-NW compounds from which the highest possible specific capacitance has been obtained by extrapolating this linear fit to n1/ 2 ¼ 0. The highest specific capacitance obtained for NiCo-NP and NiCo-NW compounds are1239.7 F g
1 and 979.5 F g
1 respectively. It is evident from Fig. 6(c) and (d) that the data have been fitted well only at lower scan rate because at higher scan rate the linear behaviour deviates due to the ohmic drop and irreversible redox reaction [53]. Eventually, the linear plot of specific capacitance vs n
1/2 (Fig. 6d) provides the value of specific capacitance dedicated by the outer surface at higher scan rate (n/∞) which depicts the total surface controlled capacitance. The estimated total specific capacitance and outer surface capacitance for the NiCo-NP compound are 1239.7 F g
1 and 174.7 F g
1 respectively. The value of specific capacitance due to the redox reaction i.e., contributed by

Fig. 6. (a) log (ip) vs log (n) plot of NiCo-NW and NiCo-NP compounds, (b) i(V)/n1/2 vs n1/2 plot of NiCo-NP and NiCo-NW compounds, (c) specific capacitance vs v1/2 plots of both compounds and (d) specific capacitance vs v
1/2 plots established from CV curve at different scan rates for measuring the supercapacitive nature.

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the inner surface of electrode material is obtained by subtracting the outer surface capacitance from total surface capacitance, is 1065 F g
1. Similarly, for NiCo-NW compound the total specific capacitance is 979.5 F g
1 which is very close to the highest specific capacity value obtained from the CV curve. The outer surface contribution towards specific capacitance and the inner surface contribution due to the redox reaction are 308.8 F g
1 and 670.6 F g
1 respectively. So, in both cases, it is worth noting that the charge storage mechanism is mainly dominated by the faradaic reaction because in both compounds the inner surface contribution is very high than that of the outer surface contribution towards the specific capacitance. The inner charge contribution is originating from the internal active sites like internal region of voids, grain boundaries, pores, dislocation, cracks etc. It strongly proves that there are 93% and 68% strong contributions of redox reaction via inner active sites to the high specific capacitance of NiCo-NP and NiCo-NW compounds. The areal capacitance of the electrode material has been measured from CV. The areal capacitance in different scanning rates is calculated from the CV curve according to the following equation [2e10,47,]:

Cm ¼

i 2av

(8)

which is the same as Eq. (2) except for mass (m) of the electrode is replaced by area (a) of the active electrode. A comparison of the areal capacitance of NiCo-NP and NiCo-NW compounds are shown in Fig. 7(a). The maximum areal capacitance of NiCo-NP and NiCo-NW are 5.96 F cm
2 and 4.93 F cm
2 respectively. This elevated areal capacitance is achieved with mass loading of 2 mg cm
2. The electrochemical performance of the sample is further examined by charge-discharge testing. The discharge capacitance is calculated from the charge-discharge testing. Fig. 7(b) and (c) show

the GCD curves of NiCo-NP and NiCo-NW electrodes at different current densities. The difference in discharge curve nature between NiCo-NP and NiCo-NW electrodes are noticeable here. Consistent with the CV result, NiCo-NP and NiCo-NW compounds demonstrate the maximum specific capacitance of 943.327 F g
1 and 782.358 F g
1 at 4 and 8 A g
1 respectively. The discharge curve of NiCo-NP consists of two different regions: (i) a small but rapid IR drop due to the resistance between electrode-electrolyte and internal resistance and (ii) a curved region due to redox activity. It is also to be noted that the specific capacitance of NiCo-NP is 119.27 F g
1 at 20 A g
1 which is ~12% that of 4 A g
1 (Fig. 7(d)). The corresponding value for NiCo-NW is 642.2 F g
1 and the capacitance retention is ~82% for this material (Fig. 7(d)). Thus the capacitance retention of NiCo-NW electrode is much better than that of NiCo-NP electrode. EIS study of these two samples have been discussed in Supporting Information (S·I) file. For further investigation of electrochemical performance of NiCo-NP and NiCoNW electrodes in three-electrode system, two salient parameters like energy density and power density have been characterized. The energy and power densities are calculated from the GCD curve at different current densities applying the following equations:

Energy density ðEd Þ ¼

1 CðDVÞ2 7:2

(9)

where C is the specific capacitance at a particular current density, DV is the potential drop during discharge, and

Power density ðPd Þ ¼

Ed  3600 t

(10)

Here P is the power density, t is the duration of discharge time. Ed is expressed in the unit of Wh Kg
1 and Pd is expressed in the unit of W Kg
1. The maximum energy density is acquired by NiCoNW as 66.85 Wh Kg
1 at a power density of 2455.8 W kg
1 and for

Fig. 7. (a) The areal capacitance of NiCo-NP and NiCo-NW compounds as a function of scan rate. GCD curves of the (b) NiCo-NP and (c) NiCo-NW electrodes at different current densities upto 20 A g-1, (d) corresponding specific capacitance as a function of current density.

M. Chatterjee et al. / Journal of Alloys and Compounds 821 (2020) 153503

9

Fig. 8. Electrochemical measurement of NiCO//AC ASC, (a) CV curves at different scan rates ranging from 2 to 100 mV s
1 with voltage window 1.6 V, (b) CV curves for different potential windows ranging from 1 to 1.8 V at scan rate 100 mV s
1, (c) GCD curves at different specific currents ranging from 1 to 10 A g
1, (d) schematic diagram of ASC, (e) Ragone plot, (f) Cyclic stability for 5000 cycles at a current density of 6.66 A g
1, (g) Nyquist plot of impedance from 0.1 Hz to 100 kHz curves of NiCO//AC ASC, inset shows the corresponding equivalent circuit.

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M. Chatterjee et al. / Journal of Alloys and Compounds 821 (2020) 153503

NiCo-NP, it is 103.75 Wh Kg
1 at a power density of 1356.2 W kg
1. To verify the feasibility of NiCo2O4 compound as an asymmetric pseudocapacitor material for energy application, an asymmetric supercapacitor (ASC) has been fabricated by assembling NiCo-NP as the positive electrode, activated carbon as the negative electrode and filter paper as a separator in 1 M Na2SO4 electrolyte. The CV curves of asymmetric supercapacitor with scan rate ranging from 2 to 100 mV s
1 are shown in Fig. 8(a). Fig. 8(b) shows the CV measurement of asymmetric supercapacitor at 100 mVs
1 operated at various operational voltages from 1 V to 1.8 V. The resembles of CV curve certain the fact that without any degradation the device can perform within the maximum potential window. The arranged prototype ASC can exhibit high potential window of 1.8 V which is beneficial for using the device in the potential application. The area under the curve increases with the increase of scan rate without any distortion, which illustrates the fast charge-discharge property. The maximum specific capacitance offered by the ASC is 208.8 F g
1 at a scan rate of 2 mV s
1 within a potential window 1.6 V. Fig. 8(c) shows representative GCD curve of asymmetric supercapacitor at current densities of 0.66 A g
1 to 6.66 A g
1. From the GCD the maximum specific capacitance provided by ASC at a current density of 0.66 A g
1 is 167.5 F g
1. The maximum specific energy delivered by the asymmetric device is 59.56 Wh Kg
1. The fabricated ASC device is shown in Fig. 8(d). Fig. 8(e) represents the Ragone plot which shows the variation of energy density with power density. The device offers a maximum power density of 3403 W kg
1 with a high energy density of 4.197 Wh Kg
1 which is superior to the typical NiCo2O4 [60e67] or nickel-cobalt oxides [3]. This result manifests revamped energy density and power density compared with nickel-cobalt oxide asymmetric device [60,61], NiCo2O4 nanosheets-decorated Cu/CuOx asymmetric device [62], Grapheneenickel cobaltite nanocomposite asymmetrical supercapacitor [63], NiCo2O4/NiO [64], mesoporous spinel NiCo2O4 [65], electrodeposited mesoporous NiCo2O4 [66], hierarchical Dandelion-like molybdenum-nickel-cobalt ternary oxide [67] as well as NiCo2O4 nanosheets [68]. Long term cyclic performance is a very critical parameter to measure the practical value of any supercapacitor device. This cycling property (Fig. 8(f)) is evaluated by repeated chargedischarge measurement up to 5000 cycles at a current density of 6.66 A g
1. The specific capacitance reaches maximum 115% retention during 2300 cycle. It is because of the activation of NiCO// AC ASC surface due to repeated charge discharge cycles. This improves the accessibility of electrolyte ions into the small pores of materials. The improved accessibility of ions increase the number of redox reactions and thus increases the capacitance value. Up to 2800 cycle, the device exhibits greater than 100% retention. After 2800 cycles the capacitance retention falls to 80%. This decrease in capacitance retention due to mechanical expansion of NiCo2O4 nanowires is due to the continuous ion insertion/desertion process or dissolution of some amount of NiCo2O4 in electrolyte. After 5000 cycles 77% capacitance retention can be maintained. Metal oxide electrode materials generally evince inferior cycle stability and service life due to conglomeration particles but here the fabricated device presents excellent cycling stability. EIS for the device was performed at 0 V within frequency 0.01 Hze105 Hz by applying AC perturbation potential of 10 mV. Fig. 8(g) represents the Nyquist plot of ASC device whose intercept at x axis gives the total internal resistance (Rs) value and the diameter of the semicircular arc gives the charge transfer resistance (Rct), while the straight line in low frequency region indicates the frequency dependent diffusion control Warburg impedance (WO). This depressed semicircular arc in the high freq region implies that materials is highly porous and having enormous active sites. In the equivalent circuit (shown inset of Fig. 8(g)) a constant CPE is used in

parallel with the Rct . The device offered low contact resistance of 0.25 U and diffusion resistance of 8.17 U. These lower values indicate fast charge transfer and better power delivery. In the high frequency region the capacitive mechanism is mainly dominated by the double layer formation on the interface of the electrode and electrolyte. In the Nyquist plot the constant phase element (CPEDL) represents this capacitive charge storage mechanism. The high value of CPEDL(CPEDL-T ¼ 0.0043 and CPEDL-P ¼ 0.71) suggests good storage capability of the device. 4. Conclusion NiCo2O4 nanowires (NiCo-NW) and porous nanoparticles (NiCoNP) have been synthesized using two simple chemical approaches for fabrication of electrode materials with superior quality for advanced asymmetric supercapacitor application. It has been demonstrated that due to the mesoporous nature, fast and convenient electron-ion transport and redox reaction, both NiCo-NW and NiCo-NP compounds show excellent electrochemical performance. NiCo-NP and NiCo-NW exhibit enormous high areal capacitance of 5.96 F cm
2 and 4.93 F cm
2 with a 2 mg cm
2 mass loading. The low dimensional NiCo-NP exhibits superior specific capacitance of 1066.03 F g
1 which is better than that of NiCo-NW (880.72 F g
1 at a scan rate 2 mV s
1), though the capacitance retention of NiCo-NW (~82%) compound is superior to NiCo-NP (~12%). However, the NiCo-NP asymmetric supercapacitor exhibits maximum specific energy of 59.56 Wh Kg
1 and maximum power density of 3403 W kg
1 with a high energy density of 4.197 Wh Kg
1. The device preserves 77% of its initial capacitance after 5000 cycle which confirms good cyclic stability and suitability of the device for supercapacitor application. This excellent nickel cobaltite spinel with mesoporous nanoparticle morphology excels over most of the metal oxide electrochemical capacitors. Due to the lower value of Rct, better ion diffusion into the electrode materials NiCo-NW shows better retention capability than NiCo-NP. But the charge storage capability of NiCo-NP is better than NiCo-NW. Although the nanoparticle system shows higher charge transfer resistance compared to the nanowire system the good charge storage mechanism of the nanoparticle system ensures higher specific capacitance for the nanoparticle sample. These results signify that these two materials could be used in high-performance electrochemical capacitors as well as energy storage devices. Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. CRediT authorship contribution statement Mahasweta Chatterjee: Conceptualization, Methodology. Samik Saha: Investigation. Sachindranath Das: Writing - review & editing. Swapan Kumar Pradhan: Writing - review & editing, Supervision. Acknowledgement M. Chatterjee and S. K. Pradhan thank the Department of Science and Technology (DST) for creating the XRD facility under the FIST programme and the University Grants Commission (UGC) India for granting the ‘‘Centre of Advanced Study’’ programme under the thrust area ‘‘Condensed Matter Physics including Laser Applications’’ to the Department of Physics,The University of Burdwan, under the financial assistance of which the work has been carried

M. Chatterjee et al. / Journal of Alloys and Compounds 821 (2020) 153503

out. M. Chatterjee wants to thank the Department of Science and Technology (DST) for providing the INSPIRE research fellowship to carry out the research work. S. Saha is grateful to CSIR, Govt. of India, for financial support. S. Das is thankful to the Department of Science and Technology (DST), Government of India, for providing research funding through the ‘INSPIRE Faculty Award’.

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Appendix A. Supplementary data [28]

Supplementary data to this article can be found online at https://doi.org/10.1016/j.jallcom.2019.153503. [29]

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