Enhanced performance on capacity retention of hierarchical NiS hexagonal nanoplate for highly stable asymmetric supercapacitor

Accepted Manuscript Enhanced performance on capacity retention of hierarchical NiS hexagonal nanoplate for highly stable asymmetric supercapacitor S. Harish, Nirmalesh Naveen, R. Abinaya, J. Archana, R. Ramesh, M. Navaneethan, M. Shimomura, Y. Hayakawa PII:

S0013-4686(18)31449-X

DOI:

10.1016/j.electacta.2018.06.161

Reference:

EA 32154

To appear in:

Electrochimica Acta

Received Date: 15 February 2018 Revised Date:

11 June 2018

Accepted Date: 24 June 2018

Please cite this article as: S. Harish, N. Naveen, R. Abinaya, J. Archana, R. Ramesh, M. Navaneethan, M. Shimomura, Y. Hayakawa, Enhanced performance on capacity retention of hierarchical NiS hexagonal nanoplate for highly stable asymmetric supercapacitor, Electrochimica Acta (2018), doi: 10.1016/j.electacta.2018.06.161. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

ACCEPTED MANUSCRIPT

Enhanced Performance on Capacity Retention of Hierarchical NiS Hexagonal Nanoplate for Highly Stable Asymmetric Supercapacitor S. Harisha#, Nirmalesh Naveenb#, R. Abinayaa,c, J. Archanac, R. Rameshd, M. Navaneethanc*,

Research Institute of Electronics, Shizuoka University, 3-5-1 Johoku, Naka-ku, Hamamatsu, Shizuoka 432-8011, Japan b

Department of Printed Electronics Engineering, Sunchon National University, Chonnam 57922, Korea

c

SC

a

RI PT

M. Shimomurae, Y. Hayakawaa*

Department of Physics and Nanotechnology, SRM Institute of Science and Technology, d

M AN U

Kattankulathur, Chennai, 603203, Tamil Nadu, India.

Department of Physics, Periyar University, Salem, 636011, Tamil Nadu, India. e

Graduate school of Science and Technology, Shizuoka University, 3-5-1 Johoku, Naka-Ku, Hamamatsu, Shizuoka 432-8011, Japan.

#Contributed Equally

*Dr. M. Navaneethan,

TE D

Corresponding author

Research Assistant Professor

SRM University.

EP

Research Institute and Department of Physics and Nanotechnology

AC C

**Prof. Y. Hayakawa

Research Institute of Electronics, Shizuoka University, 3-5-1 Johoku, Naka-ku, Hamamatsu, Shizuoka 432-8011, Japan.

E-mail: [email protected] Tel: +81 53 4781338; Fax: +81 53 4781338

1

ACCEPTED MANUSCRIPT Abstract Low energy density of the supercapacitors is considered as a roadblock for its application in or as a primary power source. While, utilization of high energy density battery-type electrode materials in an asymmetrical configuration was expected to resolve

RI PT

this hurdle, however, its inferior rate performance and poor cycling stability hinder the overall device performance. Incomplete utilization of active material at elevated current density was identified as the root for poor rate performance. Herein, we developed a

SC

hierarchical NiS microspheres build by the self-assembly of hexagonal nanoplates via trimethylamine (TEA) assisted hydrothermal method. The optimized sample exhibited a

M AN U

superior specific capacitance of 606 C/g at 0.5 A/g. More interestingly, the electrode was able to retain 50 % (302 C/g at 20 A/g) of its maximum capacity even when the current density was multiplied 40-fold relative to 18 % (50 C/g at 20 A/g) shown by control sample prepared without TEA. Excellent rate performance of the electrode could be attributed to the increment

TE D

in the electrolyte-accessible surface area by morphological modifications. Owing to its porous nature, optimized sample was able to retain 93 % of its original capacity at the end of 2000 continuous cycles of charge-discharge. Furthermore, an asymmetric supercapacitor with

EP

NiS-C as the positive electrode and activated carbon as the negative electrode delivered a high energy density of 35.07 Wh/kg at a power density of 0.420 kW/kg within an operating

AC C

voltage window of 1.5 V.

Keywords: Supercapacitor, Power density, Hierarchical, NiS, Charge-discharge, Electrode.

2

ACCEPTED MANUSCRIPT 1. Introduction Supercapacitors (SCs) or electrochemical capacitors have recently gained enormous attention from researchers for their unique features like low cost, safety, long cycle life and relatively high energy density than conventional capacitors and high power density than

RI PT

batteries [1-4]. Therefore current researches are focused on improving the energy density of the SCs without compromising the cyclic stability and rate performance via unique material design. Based on the type of charge storage mechanism, the electrode materials for SC can be

SC

broadly classified into two types, non-faradaic or electric double layer capacitive (EDLC) materials (activated carbon, carbon nanotubes, graphene etc.) and faradaic materials [5]. A

M AN U

misunderstanding has been prevailing among the researchers while classifying and treating a redox based electrode material [6]. Materials that store charge via faradaic process, yet, exhibits the virtue of a capacitive material are derived as pseudo-capacitive materials (RuO2 [7], MnO2 [8], polyaniline (PANI) [9], polypyrrole (PPy) [10] etc.) and those that are

TE D

potential dependent is known as battery-type materials.

Hybridizing a battery-type electrode material of very high energy density (ca. 100 Wh/kg) [11] with a capacitive material of extreme power density in an asymmetrical

EP

configuration could circumvent the shortcomings of a supercapacitor [12]. However, poor cycle life and rate performance of a battery-type electrode material hinders the realization of

AC C

a practical asymmetrical capacitor in high power applications such as electric vehicles [4]. In this regard several battery-type materials predominantly transition metal oxides (Co3O4 [13], NiO [14], Fe3O4 [15], CuO [16] etc.), hydroxides (Ni(OH)2 [17], Co(OH)2 [18], Ni-Co(OH)2 [19] etc.) and their sulfides (CoS [20], NiS [21-26], CuS [27] etc.) have been studied extensively by researchers. Low electronic conductivity and/or inferior material design incapable of buffering the volume change during ion insertion/exertion are pointed out as the major failure mechanism. 3

ACCEPTED MANUSCRIPT NiS is highly desirable for its large theoretical capacity (1060 C/g), electronic conductivity [28, 29], well-known redox activity of Ni2+ in alkaline electrolytes [26] and low cost. Previously, Wei et al. [21] synthesized α-NiS hollow spheres exhibited a maximum specific capacity of 215 C/g or 717.3 F/g at 0.6 A/g, that degraded to 37.6 C/g or 125.4 F/g

RI PT

when the current density was increased to 6 A/g. NiS2 nanocubes synthesized via microwave assisted method by Pang et al. [22] delivered a specific capacity of 278 C/g or 695 F/g at 1.25 A/g and as the current density raised by 10-fold the specific capacitance dropped to 22.7%

SC

(158 F/g at 12.5 A/g). While, hierarchical flower-like β-NiS synthesized by Yang et al. [23] displayed a high specific capacity of 428.8 C/g or 857.7 F/g at 2 A/g declined to half of its

M AN U

capacity at 5 A/g. At the end of 1000 cycles the material exhibited a 60 % capacity loss. Despite the partial success, poor rate performance and structure instability of NiS has limited its application. Incomplete utilization of the active material during rapid charge-discharge could be attributed to the inferior capacitance retention, especially at high current rates [28].

TE D

Enhancing the electrolyte-accessible surface area of electrode by tuning the morphology of nanomaterials with novel hierarchical architectures will be the key for developing a high power and energy materials. Additionally, hierarchical structure with

EP

enormous internal voids could buffer the volume change during extended cycling. To achieve high surface /volume ratio of nanoparticles numerous strategies were adapted such as ball

AC C

milling [29], decorating nanoparticles over high surface area carbon materials [30-34], hard or soft template assisted synthesis [35-38], use of surfactants [39] etc. The surfactant additives were proved to be of great effect on controlling the size and morphology of materials. Commonly used surfactants are phosphates, thiols, biomolecules, amines and other reducing agents [40-42]. Among them, amines serve as an effective capping agent for removing surface defects and to control the nucleation, growth, and alignment of crystals [43]. Triethylamine (TEA) is a successful capping agent owing to its good anionic interaction with 4

ACCEPTED MANUSCRIPT metal sulfides and control of morphology. Herein, we report a growth of three dimensional hierarchical porous microspheres constructed from NiS hexagonal nano-plates via a TEA assisted hydrothermal synthesis method. The effect of TEA concentration on NiS hierarchical structure formation and phase

RI PT

change was investigated by diffraction and imaging techniques. Electrochemical investigations revealed the sample prepared with optimum amount of TEA exhibited

SC

remarkable charge storage and excellent rate performance.

2.1. Chemicals and materials

M AN U

2. Materials and Methods

All the chemicals were of analytical grade and used without further purification. Nickel acetate (C4H14NiO8, 98 %; Wako, Japan), thioacetamide (C2H5NS, 99 %, Wako, Japan) and triethylamine (C6H15N, 99 %; Wako, Japan).

TE D

2.2. Synthesis of hierarchical NiS nanostructures

In a typical synthesis, 0.1 mol/L nickel acetate was dissolved in 100 mL of deionized water and allowed to stir for complete dissolution of the compound. Triethylamine was added

EP

to the above solution as a capping agent at various concentrations (0.25, 0.5, 0.75 and 1 mL). Then, 0.1 mol/L of thioacetamide was added to the above solution and the reaction was

AC C

allowed to stir for 10 h. Finally, the mixture was transferred to a Teflon-lined autoclave and maintained at 150 ºC for 10 h. The obtained precipitate was separated by centrifugation, washed with deionized water and ethanol for several times. The obtained product was dried at 80 ºC for 1 h. The resulting samples were termed as NiS-A for 0.25 mL, NiS-B for 0.50 mL, NiS-C for 0.75 mL and NiS-D for 1 mL, respectively. A control sample was prepared following similar procedure without TEA addition and labeled as NiS-P.

5

ACCEPTED MANUSCRIPT 2.3.

Characterization The obtained products were characterized by X-ray Diffraction (XRD) using a

Rigaku X-ray diffractometer (RINT 2200, Japan) with Cu-Kα radiation and 0.02°/s step interval. The morphology and particle size of the products were assessed by Field emission

RI PT

Electron Microscope (FESEM) (JEOL JSM 7001F microscope) at an accelerating voltage of 15 kV and High-Resolution Transmission Electron Microscope (HR-TEM) (JEOL JEM 2100F microscope) at an accelerating voltage of 200 kV. The samples were initially dispersed

SC

in ethanol using ultrasonication and drop cast onto a holey carbon-coated copper grid. X-ray Photoelectron Spectrum (XPS) were recorded by a Shimadzu ESCA 3400.

M AN U

The working electrode was prepared by mixing active material (80 %), acetylene black (10 %) and poly(vinyledene fluoride) (10 %) in N-methyl-2-pyrrolidone. The mixture was casted onto a Ni foil current collector (1 cm×1 cm) of 0.25 mm thickness (Alfa Aesar) and dried 4 h at 100 °C for solvent removal. The mass of the loaded samples lies within a

TE D

range of ca. 0.4–0.5 mg measured using a Shimadzu analytical balance of accuracy 0.01 mg and thickness ca. 13-15 µm measured using a screw gauge of least count 0.001 cm. Cyclic Voltammetry (CV), Galvanostatic Charge-Discharge (GCD) and Electrochemical Impedance

EP

Spectroscopy (EIS) were performed using a CHI 661C electrochemical workstation employing a standard 3-electrode cell configuration with a platinum wire as a counter

AC C

electrode and Hg/HgO as a reference electrode. The measurements were performed in aqueous 3 M KOH electrolyte at ambient conditions within 0.1–0.65 V potential window. Impedance spectroscopy was conducted within a frequency range of 1 Hz – 500 kHz with modulating voltage amplitude of ±10 mV at their respective open circuit potentials (OCP). A full-cell was constructed by sandwiching the positive and negative electrode soaked in a 3 M KOH solution separated by a PTFE membrane filter.

6

ACCEPTED MANUSCRIPT 3. Results and discussion Structural and elemental analysis The effect of TEA amount on the crystal structure of nickel sulfides was examined

RI PT

using XRD measurements. NiS is known to crystallize in two different phases a room temperature β-phase and high temperature α-phase. β-NiS phase crystallizes in a rhombohedral crystal structure with R3m space group has poor electronic conductivity while α-NiS that crystalizes in a hexagonal crystal structure with space group P63/mmc has metallic

SC

like conductivity [26]. XRD patterns of the samples reveal the presence of more than one

M AN U

phase, as shown in figure 1. XRD pattern of the control sample exhibited diffraction peaks corresponding to both hexagonal α-NiS phase (JCPDS card No. 02-1280) and rhombohedral β-NiS phase (JCPDS card No. 12-0041) [26]. From the relative peak intensity, α-NiS was found to be the prominent phase. Similar observation made from NiS-A and NiS-B implies invariance in crystal structure at low concentrations of TEA (0.25 mL and 0.5 mL).

TE D

Appearance of new peaks for the samples prepared using 0.75 (NiS-C) and 1 mL (NiS-D) of TEA suggest the formation of a secondary phase. Crystal structure of NiS-C and NiS-D were composed of a mixture of α-NiS phase and cubic Ni3S4 phase (JCPDS card no.30-0863) [36].

EP

The presence of less conductive secondary phases like Ni3S4 along with metal-like

AC C

conductive α-NiS phase could affect the intrinsic electrical conductivity of the compound. Peak broadening noticed in samples prepared at high conc. of TEA suggests a reduction in the average crystallite size of the compounds. Chemical state and composition of the synthesized products were identified using

XPS. Figure 2(A) displays the XPS survey spectra of the samples (a) NiS-P, (b) NiS-A, (c) NiS-B, (d) NiS-C and (e) NiS-D. Strong peaks of Ni 2p and S 2p suggest the formation of NiS. Peaks corresponding to N and O have originated from the residual TEA and surface adsorbed water molecules. The chemical state of Ni ions was determined from the 7

ACCEPTED MANUSCRIPT high-resolution Ni 2p spectra, shown in figure 2(B). The appearance of two major peaks at 853.4 eV (Ni 2p3/2) and 870.6 eV (Ni 2p1/2) with a spin orbital splitting energy of 17.2 eV are characteristics of Ni2+ species [44]. For sample NiS-C and NiS-D, a prominent shoulder peak

ascribed to higher oxidation state of Ni (3+) ions as in Ni3S4. Morphology Analysis

RI PT

appears on the higher binding energy side of Ni 2p3/2, (Denoted by * symbol) which could be

Surface textural modifications produced by different TEA concentration were

SC

analyzed using FE-SEM imaging technique. Figure 3 shows the low and high magnification images of (a) and (b) NiS-P, (c) and (d) NiS-A, (e) and (f) NiS-B, (g) and (h) NiS-C and (i)

M AN U

and (j) NiS-D respectively. FESEM images of the control sample revealed a featureless agglomerated compact morphology. Introduction TEA resulted in a slow evolution of spherical shaped particles (figure 3C). A higher magnification image (Fig. 3(d)) reveals that spherical particles were composed of smaller plate like primary particles. At slightly higher

TE D

concentration of TEA (0.5 mL), the plate-like particles transformed into well-defined NiS hexagonal nanoplates (HNPs), conforming to the crystal structure of α- NiS (figure 3 (e) and (f)). The thickness of HNPs was measured to be ca. 170 nm while width and height range in

EP

few hundreds of nanometer. Well-defined hierarchical microspheres constructed from HNPs building blocks were found at higher concentration of TEA (figure 3(g)). A closer look at the

AC C

high magnification image (figure 3(h)) reveals that microspheres were generated by intersection of HNPs via self-assembly producing enormous internal voids. While, interconnected HNPs could behave as a conductive pathway for rapid electron transfer, the pores could facilitate electrolyte inundation to the bulk of the material. Size controlling aspect of the capping agent could be understood from the reduced particle dimensions (thickness of NiS-C ca. 50 – 70 nm). Further increase of TEA conc. (>0.75mL) did not yield any distinct changes in the morphology (figure 3 (i) and (j)) compared to NiS-C, except for the growth of 8

ACCEPTED MANUSCRIPT secondary particles, as highlighted in figure 3(j). Therefore the optimum amount of TEA for the synthesis of uniform hierarchical NiS HNPs was determined to be 0.75 mL. A detailed particle structure analysis was performed using HR-TEM imaging technique. Figure 4 shows the TEM and HR-TEM images of NiS-P ((A) and (B)) and NiS-C

RI PT

((C) and (D)), respectively (TEM images of other samples were provided in supplementary information (SI), figure S1). Agglomerated compact nature of NiS-P was once again substantiated from the TEM image, see figure 4(A). High resolution TEM image (figure

SC

4(B)) reveals diffraction fringes that reflect the polycrystalline nature of the sample. The lattice spacing measured from the image matches well with the crystallographic (100) plane

M AN U

of α-NiS, in accord with XRD studies. Figure 4C displays the hexagonal geometry of the nanoplates constructed via TEA surface passivation. Interestingly the intersection of HNPs has led to the formation of nano-pores or channels, as shown in figure 4(D). The lattice spacing calculated from the fringe distance coincides well with the (101) plane of α-NiS.

TE D

Above results once again confirm that hexagonal α-NiS was the prominent phase. Based on the above results, synthesis and growth mechanism of the hexagonal nanoplates could be proposed as follows. At initial stage of the synthesis, Ni2+ ion was

EP

passivated by complexation reaction with the amine groups of TEA (TEA-Ni2+). During subsequent hydrothermal process, thioacetamide decomposes in water to give NH3 and H2S,

AC C

which readily reacts with TEA-Ni2+ complex to form hexagonal α-NiS crystallites. Ni(CH3COO)2 + TEA → TEA-Ni(CH3COO)2 (complex)

(1)

C2H5NS + 2H2O → H2S + NH3 + CH3COOH

(2)

TEA-Ni(CH3COO)2 (complex) + H2S → NiS + TEA + 2CH3COOH

(3)

The higher surface energy of the smaller crystallites drives them to coalesce for energy minimization, concomitantly self-assembling into a larger stable hexagonal nanoplate. The growth mechanism of the hierarchical NiS HNPs was graphically illustrated in figure 5. 9

ACCEPTED MANUSCRIPT Results clearly elucidate the role of TEA in tailoring the microstructure and surface morphology of the samples. Electrochemical Characterization TEA surface tailored samples were subjected to various electrochemical studies to

RI PT

analyze the impact of electrode interfacial properties on charge storage. Voltammetry is considered as a powerful tool due to its sensitivity towards electrode/electrolyte interfacial reactions. Figure 6 compares the cyclic voltammetry curves of samples recorded at 20 mV/s

SC

scan rate within a potential window of 0.1 to 0.65 V vs. Hg/HgO in 3 M KOH electrolyte. It is to be noted here, that the current values were normalized based on the electrode weight.

M AN U

The inset of figure 6 shows the magnified portion of the CV curve corresponding to the capacitive current. In general, double layer capacitive current is directly proportional to the interfacial surface area. An increase in capacitive current from NiS-P to NiS-D suggests the enhancement in interfacial area between electrode and electrolyte. A distinct pair of redox

TE D

peaks could be observed in the CV curve for all the samples suggesting a faradaic type redox reaction and the area under the peaks indicates its major share in charge storage, typical of a battery-type electrode material. The appearance of redox peaks was due to the reversible

EP

redox transition of Ni ions (Ni2+↔Ni3+) in alkaline solution, consistent with previous reports [21-24]. Cathodic peak potential tends to vary for the samples, while anodic peak potential

AC C

remains invariant (ca. 0.51 V). The relative shift in the reduction peak resulting in a larger peak separation for NiS-C and NiS-D implies a greater polarization resistance of the electrodes. The higher over-potential for reduction observed for NiC and NiS-D could be due to the presence of less conductive secondary phases, as described in the XRD section. Visibly, NiS-C and NiS-D demonstrated the highest current response at all regions of the CV curve. The CV curves of NiS-P, NiS-A, NiS-B, NiS-C and NiS-D, recorded at various (5, 10, 20, 30, 40, 50, 60, 70, 80, 90 and 100 mV/s) scan rates is shown in figure S2 of SI. A linear increase 10

ACCEPTED MANUSCRIPT of peak current with square root of scan rate and retention of CV shape even at high scan rates depicts the rapid ion diffusion and facile charge transfer in the material. The anodic and cathodic peaks were found to shift towards right and left respectively with an increase in scan rate. This shift could be ascribed to the polarization of the electrodes at different scan rates.

equation [45]. 



 = ×    

RI PT

The specific capacities of the samples were calculated from the CV curves following bellow

(4)

SC

Where SC is the specific capacity in C/g,  is the scan rate in mV/s, m is the mass of the

M AN U

active material in g, the integral portion of the equation was obtained directly from the cathodic area of the CV curve. SC of the samples NiS-P, NiS-A, NiS-B, NiS-C and NiS-D calculated at 5 mV/s sweep rate were 289 C/g (525 F/g), 415 C/g (754 F/g), 473 C/g (860 F/g), 622 C/g (1130 F/g) and 634 C/g (1152 F/g) respectively. It is to be noted here, only for the sake of comparison with literature values the specific capacitance was given in F/g metric.

TE D

The specific capacity was found to increase progressively with the transformation of irregular agglomerated morphology of NiS-P to a 3D-hierarchial nanostructure with abundant electrolyte diffusion channels, as in the case of NiS-C. The agglomerated compact

EP

morphology prevents the accessibility of interior active sites to the electrolyte ions under

AC C

given charge-discharge period thereby limiting the gravimetric specific capacity (as in NiS-P). On the contrary, a 3D-hierarchial nanostructure, as in the case of NiS-C, has enormous voids enabling sufficient wetting of the electrode and in return shortened diffusion length. Therefore, electrolyte ions could rapidly access the inner-most active sites easily contributing to a very high specific capacity. Since, the surface morphology of NiS-D has no significant change as compared to NiS-C, correspondingly there was no significant improvement in the specific capacity was observed. Therefore, the enhancement in SC can be directly related to the surface characteristics of the NiS brought out by TEA incorporation. The enhancement in 11

ACCEPTED MANUSCRIPT the specific capacity with increase in TEA amount was shown in figure S3 of SI. Nearly, a two fold increase in the specific capacity was noticed for NiS-C in contrast to NiS-P. These values are higher than those reported earlier for nickel sulfide based materials, see Table 1. To more understand the striking difference in SC of NiS-C relative to NiS-P, a method followed

RI PT

by Ardizzone et al. [46] was employed to estimate the contribution to the total charge stored by capacitive (non-fardaic) and diffusion dependent faradaic process. A plot was drawn between specific charge and inverse of square root of scan rate for NiS-P and NiS-C, see

SC

figure 7(A) and (C). It is to be noted that the charge was calculated for both the electrodes of equivalent mass and area from the cathodic part of the CV curve to avoid the contribution

M AN U

from water oxidation. The amount of charge stored via double-layer formation can be determined from the intercept of the line on the Y-axis corresponding to infinite scan rate (figure 7(B) and (D) for NiS-P and NiS-C respectively). NiS-C stores ca. 0.0546 C/cm-2 via double-layer formation a two-fold higher than that of NiS-P (ca. 0.0286 C/cm2). For NiS-C at

TE D

5 mV/s scan rate nearly ca. 72 % of the total charge stored is contributed by faradaic process that involves a semi-infinite diffusion of electrolyte ions, while only 54 % was calculated for NiS-P. A remarkable difference could be observed at 100 mV/s, where the diffusion

EP

contribution remains to be ca. 39 % for NiS-C in contrast to ca. 18 % observed for NiS-P. These results signifies that enhancement of electrode/electrolyte interfacial area not only

AC C

increases the capacitive charge storage but also improves the active material utilization by rapid ion diffusion. A decline in specific capacity with increase in scan rate was observed (see SI figure S4), that is a characteristic of diffusion controlled rate kinetics of the electrode process. The power characteristics of the samples were examined by the galvanostatic charge-discharge (CD) study. Figure 8 (A-E) shows the CD profile of NiS-P, NiS-A, NiS-B, NiS-C and NiS-D respectively at various current densities like 0.5, 1, 2, 3, 4, 5, 10 and 20 A/g. 12

ACCEPTED MANUSCRIPT Appearance of long charge and discharge plateau’s in CD curve is typical of a battery-type charge storage, coinciding with CV results. Longer duration of discharge time means greater the charge stored. Clearly, NiS-C and NiS-D exhibits the highest discharge time at all current densities with respect to other samples. The specific capacity was calculated from the CD

 =

RI PT

study using below equation [47]. ×

(5)



SC

where, I is the current (A), td is the discharge time (s) and m is the mass of the active material (g). The specific capacity values calculated for all the samples were represented

M AN U

graphically in Fig. 8 (F). Maximum specific capacity of 273 (546 F/g), 396 (792 F/g), 448 (896 F/g), 606 (1212 F/g) and 617 C/g (1234 F/g) was demonstrated by NiS-P, NiS-A, NiS-B, NiS-C and NiS-D respectively, at a current density of 0.5 A/g. Owing to its surface characteristics, NiS-C was able to retain nearly ca. 50 % (302 C/g at 20 A/g) of its maximum SC under a 40-fold increase in current density in contrast to ca. 18 % observed for NiS-P (50

TE D

C/g at 20 A/g) and 44 % for NiS-D (280 C/g at 20 A/g). It is remarkable to note that the SC of NiS-C at 20 A/g was six times higher than NiS-P displaying excellent rate characteristics. Since, NiS-C was identified as the optimized compound other samples (NiS-A, NiS-B and

EP

NiS-D) were neglected for further studies. The ability of the porous 3D-hierarchial structure

AC C

of NiS-C to withstand the large volume change induced by continuous cycling was tested by performing 2000 CD cycles at a high current density of 10 A/g and contrasted with that of NiS-P. Figure 9 (A) displays the results of the long cycling study. During initial cycles, a capacity fading was noticed for both the samples. But, in the subsequent cycles specific capacity of NiS-C was found to increase in contrast to steep fading of NiS-P. At the end of 2000 cycles, NiS-C exhibited remarkable capacity retention of 93 % against 80 % of NiS-P. Rapid charge transfer and ion diffusion kinetics of the electrodes was analyzed using electrochemical impedance spectroscopy (EIS) technique. Figure 9 (b) shows the Nyquist 13

ACCEPTED MANUSCRIPT plot of NiS-P and NiS-C (symbols represent experimental data and the solid line represents the fitted data). Inset of the figure shows the magnified portion of the high frequency region and the Randle’s equivalent circuit that best fits the experimental data. RS is the equivalent series resistance (ESR) or solution resistance, RCT is charge transfer resistance, CPE1 and

RI PT

CPE2 corresponds to the constant phase elements representing two different types of capacitive behavior and W is the Warburg impedance. Equivalent series resistance is nothing but the summation of electrolyte resistance, charge transfer resistance between the electrode

SC

and current collector and intrinsic electrode resistance [48]. Since reactance of a capacitor is inversely proportional to frequency (XC = 1/2πfC; XC is reactance, f is frequency and C is

M AN U

capacitance) at high frequencies the current flows only through the resistor component (RS). Therefore, RS value can be determined from the intercept of the curve on the real axis at a high frequency. RS values extracted from the equivalent circuit denotes negligible difference between NiS-P (1.01 Ω·cm-2) and NiS-C (1.25 Ω·cm-2). The observed RS value range is in

TE D

agreement with the previous literature [22, 28, 49]. In the equivalent circuit constant phase element (CPE) was used in the place of an ideal capacitor as a consequence of inhomogeneity in the surface texture at the electrode-electrolyte interface and it is independent of frequency.

EP

In the equivalent circuit, CPE1 with a resistor (RCT) in parallel represent the faradaic capacitive nature that is attributed to the Ni2+/Ni3+ redox transition coupled with an

AC C

adsorption/desorption of electrolyte species (OH-) and associated charge transfer resistance across the interface respectively. The diameter of the semi-circle observed at the high-frequency region is translated as the magnitude of charge transfer resistance, presence of a depressed semi-circle in the EIS curve suggests a smaller charge transfer resistance. It was found that NiS-C has a relatively lower RCT (42.84 Ω) than NiS-P (57.13 Ω). CPE2 in the circuit could be ascribed to the double layer capacitive contribution. The n values of CPE1 and CPE2 were close to ca. 0.98, indicating a near ideal capacitive behavior. The capacitance 14

ACCEPTED MANUSCRIPT value extracted from the equivalent circuit suggests a better charge storage in NiS-C (CPE1= 1.56 mF·cm-2 and CPE2= 1.61 mF·cm-2) relative to NiS-P (CPE1= 1.45 mF·cm-2 and CPE2= 1.09 mF·cm-2). Since the EIS spectra were recorded at respective OCP’s the faradaic contribution appears to be less significant. Despite a similar electronic charge transport

RI PT

behavior of NiS-P and NiS-C a clear evidence for the superior electrochemical behavior of NiS-C could be observed from the mid-low frequency region corresponding to the ion transport or diffusional characteristics of the material. The slope of the 45° portion of curve

SC

between mid-low frequency regions gives the measure of the Warburg resistance or diffusion resistance, which is the result of the frequency dependence of ion diffusion in the electrolyte

M AN U

to the electrode surface [17]. The Warburg resistance WR extracted from the equivalent circuit suggests NiC-C (16.93 Ω Ω·cm-2) has a significantly lower diffusion resistance than NiS-P (28.77 Ω·cm-2). The straight line observed in the low frequency region is an indication of facile ion diffusion and ideal capacitive nature. The steeper line observed for NiS-C in

TE D

contrast to NiS-P that is curving towards the X-axis suggests the facile diffusion of electrolyte ions (OH-) from the bulk to the electrode surface. Evidently, the surface modification of NiS

of active sites.

EP

by TEA has enhanced the ion diffusional kinetics of the electrode enabling wider accessibility

Asymmetrical full-cell study on NiS-C

AC C

To investigate the performance of the optimized electrode material in an actual

device, it was paired with commercial activated carbon (surface area – 1666 m2/g, particle size – 2.25 μm and pore volume – 0.75 ml/g, MTI corp. USA) in an asymmetrical configuration. The specific capacity of the activated carbon was determined from the 3-electrode setup identical to that described for the case of NiS samples. The CV profile of AC scanned at various scan rates resembles a rectangular shape typical of an EDLC electrode, see Fig. S5. A maximum specific capacitance of 175 F/g was obtained at 5 mV/s scan rate 15

ACCEPTED MANUSCRIPT within a potential window of -0.8 – 0.1 V (ΔV= 0.9 V). Figure 10A compares the CV profile of AC and NiS-C at 20 mV/s sweep rate illustrating that in an asymmetrical configuration the cell could be operated safely to a maximum window of 1.5 V. The mass ratio between a

equation  ×  ×  =  =  ×  × 

RI PT

positive (mp) and negative (mn) electrode were derived according to the charge balance

(6)

Where, Q is the charge on each electrode, SC is the specific capacity (subscript indicates

SC

the type of the electrode), m is the mass and ΔV is the potential window. Mass balance ratio was determined as Mp: Mn= 1: 2.46, Mp is for NiS-C and Mn is for AC. Fig. 10B displays the

M AN U

CV curve of a NiS-C/AC full-cell recorded at various scan rates (5, 10, 25, 50, 75 and 100 mV/s). Conspicuous redox peaks observed in the CV curves even at high scan rates depict the excellent electro-activity of NiS-C. Generally, in an asymmetrical configuration battery-type electrodes will be the rate limiting electrodes owing to their diffusion limited kinetics but in

TE D

our case NiS-C was able to keep up with the capacitive electrode displaying high power characteristics. A Galvanostatic CD curve obtained at various current densities (0.5, 1, 1.5, 2, 2.5, 5 and 10 A/g) is shown in Fig. 10B. A maximum specific capacitance of 112 F/g was

EP

delivered by NiS-C/AC at 0.5 A/g current density. Excellent capacitance retention of 69% (77 F/g) was delivered when the current density was increased 20 fold (10 A/g). Energy density

AC C

(ED) and power density (PD) are the two important parameters that gauge a device performance, Fig. 10D shows the Ragone plot drawn ED vs PD for NiS-C/AC. The device delivered a highest ED of 35.07 Wh/kg at 0.420 kW/kg PD. Superiority of the device could be witnessed especially from the retention of high ED (24.4 Wh/kg) at extreme PD (5.8 kW/kg). The remarkable ED’s delivered at high rates were made possible by the enhancement of electrolyte-accessible surface area through surface tailoring of NiS samples by TEA as mentioned earlier. 16

ACCEPTED MANUSCRIPT 4. Conclusion Surface tailored NiS was prepared via TEA assisted hydrothermal method. Prepared samples exhibit a mixture of phases depending on the TEA concentration. Imaging technique reveals the stepwise transformation of agglomerated compact morphology in a pristine

RI PT

sample to a hierarchical microsphere formed by a self-assembly of HNPs with the corresponding addition of TEA amount. The sample prepared with optimum amount of TEA demonstrated a high specific capacity of 606 C/g at a current density of 0.5 A/g and was able

SC

to retain 50% (302 C/g) of its maximum capacity at 20 A/g in contrast to 18% (50 C/g) retained by NiS-P. Capacity retention test conducted for 2000 cycles revealed that NiS-C was

M AN U

able to retain 93% of its original capacity against 80% delivered by NiS-P. Additionally, the NiS-C//AC asymmetrical supercapacitor with NiS-C as the positive electrode and AC as the negative electrode exhibited an operating voltage of 1.5 V in a 3 M KOH electrolyte and the device could deliver an high energy density of 35.07 Wh/kg at 0.420 kW/kg power density.

TE D

This high energy density was not substantially reduced (24.4 Wh/kg) when the power density was increased 14 fold. Enhancement in the electrolyte accessible surface area via surface modification has enabled ideal utilization of the active materials at very high

EP

charge-discharge rates. Moreover, the porous architecture aid facile ion diffusion and

AC C

accommodates volume strain occur during extended cycling.

17

ACCEPTED MANUSCRIPT References: [1] G. Wang, L. Zhang, J. Zhang, A review of electrode materials for electrochemical supercapacitors, Chem. Soc. Rev. 41 (2012) 797. [2] A. Gonzalez, E. Goikolea, J.A. Barrena, R. Mysyk, Review on supercapacitors:

RI PT

technologies and materials, Renew Sustainable Energy Rev. 58 (2016) 1189.

[3] V. Augustyn, P. Simon, B. Dunn, Pseudocapacitive oxide materials for high-rate electrochemical energy storage, Energy Environ. Sci. 7 (2014) 1597.

SC

[4] H. Wang, H.S. Casalongue, Y. Liang, H. Dai, Ni(OH)2 nanoplates grown on Graphene as advanced electrochemical pseudocapacitor materials, J. Am. Chem. Soc. 132 (2010)

M AN U

7472.

[5] N. Kyung-Wan, K. Kwang-Heon, L. Eun-Sung, Y. Won-Sub, Y. Xiao-Qing, K. Kwang-Bum, Pseudocapacitive properties of electrochemically prepared nickel oxides on 3-dimensional carbon nanotube film substrates, J. Power Sources 182 (2008) 642.

TE D

[6] T. Brousse, D. B´elanger, J.W. Long, To Be or Not To Be Pseudocapacitive?, J. Electrochem. Soc. 162 (2015) A5185.

[7] J.H. Jang, A. Kato, K. Machida, K. Naoi, Supercapacitor performance of hydrous

EP

ruthenium oxide electrodes prepared by electrophoretic deposition, J. Electrochem. Soc. 153 (2006) A321.

AC C

[8] H. Jiang, T. Sun, C. Li, J. Ma, Hierarchical porous nanostructures assembled from ultrathin MnO2 nanoflakes with enhanced supercapacitive performances, J. Mater. Chem. 22 (2012) 2751.

[9] K. Wang, J. Huang, Z. Wei, Conducting Polyaniline nanowire arrays for high performance supercapacitors, J. Phys. Chem. C 114 (2010) 8062. [10] Y. Shi, L. Pan, B. Liu, Y. Wang, Y. Cui, Z. Bao, G. Yu, Nanostructured conductive polypyrrole hydrogels as high-performance, flexible supercapacitor electrodes, J. Mater. 18

ACCEPTED MANUSCRIPT Chem. A 2 (2014) 6086. [11] X. Zhang, W. Shi, J. Zhu, D. J. Kharistal, W. Zhao, B.S. Lalia, H.H. Hng, Q. Yan, High-power and high-energy-density flexible pseudocapacitor electrodes made from porous CuO nanobelts and single-walled carbon nanotubes, ACS Nano 5 (2011) 2013.

RI PT

[12] M. Yang, Y. Zhong, J. Bao, X. Zhou, J. Wei, Z. Zhou, Achieving battery-level energy density by constructing aqueous carbonaceous supercapacitors with hierarchical porous N-rich carbon materials, J. Mater. Chem. A 3 (2015) 11387.

SC

[13] W. Du, R. Liu, Y. Jiang, Q. Lu, Y. Fan, F. Gao, Facile synthesis of hollow Co3O4 boxes for high capacity supercapacitor, J. Power Sources 227 (2013) 101.

M AN U

[14] C. Yuan, X. Zhang, L. Su, B. Gao, L. Shen, Facile synthesis and self-assembly of hierarchical porous NiO nano/micro spherical superstructures for high performance supercapacitors, J. Mater. Chem. 19 (2009) 5772.

[15] L. Wang, H. Ji, S. Wang, L. Kong, X. Jiang, G. Yang, Preparation of Fe3O4 with high

3793.

TE D

specific surface area and improved capacitance as a supercapacitor, Nanoscale 5 (2013)

[16] S.E. Moosavifard, M.F. El-Kady, M.S. Rahmanifar, R.B. Kaner, M.F. Mousavi,

EP

Designing 3D highly ordered nanoporous CuO electrodes for high-performance asymmetric supercapacitors, ACS Appl. Mater. Interfaces 7 (2015) 4851.

AC C

[17] L. Zhang, Q. Ding, Y. Huang, H. Gu, Y.E. Miao, T. Liu, Flexible hybrid membranes with Ni(OH)2 nanoplatelets vertically grown on electrospun carbon nanofibers for high-performance supercapacitors, ACS Appl. Mater. Interfaces 7 (2015) 22669. [18] L.B. Kong, M. Liu, J.W. Lang, Y.C. Luo, L. Kang, Asymmetric supercapacitor based on loose-packed cobalt hydroxide nanoflake materials and activated carbon, J. Electrochem. Soc. 156 (2009) A1000. [19] H. Chen, L. Hu, M. Chen, Y. Yan, L. Wu, Nickel–Cobalt layered double hydroxide 19

ACCEPTED MANUSCRIPT nanosheets for high-performance supercapacitor electrode materials, Adv. Funct. Mater. 24 (2014) 934. [20] Z. Yang, C.Y. Chen, H.T. Chang, Supercapacitors incorporating hollow cobalt sulfide hexagonal nanosheets, J. Power Sources 196 (2011) 7874.

RI PT

[21] C. Wei, C. Cheng, Y. Cheng, Y. Wang, Y. Xu, W. Du, H. Pang, Comparison of NiS2 and α-NiS hollow spheres for supercapacitors, non-enzymatic glucose sensors and water treatment, Dalton Trans. 44 (2015) 17278.

SC

[22] H. Pang, C. Wei, X. Li, G. Li, Y. Ma, S. Li, J. Chen, J. Zhang, Microwave-assisted synthesis of NiS2 nanostructures for supercapacitors and cocatalytic enhancing

M AN U

photocatalytic H2 production, Scientific Reports 4 (2014) 3577.

[23] J. Yang, X. Duan, Q. Qin, W. Zheng, Solvothermal synthesis of hierarchical flower-like b-NiS with excellent electrochemical performance for supercapacitors, J. Mater. Chem. A 1 (2013) 7880.

TE D

[24] T. Zhu, Z. Wang, S. Ding, J.S. Chen X.W. Lou, Hierarchical nickel sulfide hollow spheres for high performance supercapacitors, RSC Adv. 1 (2011) 397. [25] J. Yang, W. Guo, D. Li, C. Wei, H. Fan, L. Wu, W. Zheng, Synthesis and

EP

electrochemical performances of novel hierarchical flower-like nickel sulfide with tunable number of composed nanoplates, J. Power Sources 268 (2014) 113.

AC C

[26] C. Sun, M. Ma, J. Yang, Y. Zhang, P. Chen, W. Huang, X. Dong, Phase-controlled synthesis of α-NiS nanoparticles confined in carbon nanorods for high performance supercapacitors, Sci. Rep. 4 (2014) 7054. [27] H. Yu-Kuei, C. Ying-Chu, L. Yan-Gu, Synthesis of copper sulfide nanowire arrays for high-performance supercapacitors, Electrochim. Acta 139 (2014) 401. [28] C. Jiang, B. Zhao, J. Cheng, J. Li, H. Zhang, Z. Tang, J. Yang, Hydrothermal synthesis of Ni(OH)2 nanoflakes on 3D graphene foam for high-performance supercapacitors, 20

ACCEPTED MANUSCRIPT Electrochimica Acta 173 (2015) 399. [29] N.A.M. Zaid, N.H. Idris, Enhanced Capacitance of Hybrid Layered Graphene/Nickel Nanocomposite for Supercapacitors, Sci. Rep. 6 (2016) 32082. [30] Y. Li, K. Ye, K. Cheng, J. Yin, D. Cao, G. Wang, Electrodeposition of nickel sulfide on

supercapacitors, J. Power Sources 274 (2015) 943-950.

RI PT

graphene-covered make-up cotton as a flexible electrode material for high-performance

[31] A. Wang, H. Wang, S. Zhang, C. Mao, J. Song, H. Niu, B. Jin, Y. Tian, Controlled

supercapacitor, Appl. Sur. Sci. 282 (2013) 704.

SC

synthesis of nickel sulfide/graphene oxide nanocomposite for high-performance

M AN U

[32] J. Yang, X. Duan, W. Guo, D. Li, H. Zhang, W. Zheng, Electrochemical performances investigation of NiS/rGO composite as electrode material for supercapacitors, Nano Energy 5 (2014) 74.

[33] D. Chao-Shuan, C. Pei-Yi, L. Jeng-Yu, C. Shu-Wei, W. Wen-Kai, L. Ping-Hsuan, W.

TE D

Kuan-Yi, L. Tsung-Wu, Hierarchically structured Ni3S2/Carbon nanotube composites as high performance cathode materials for asymmetric supercapacitors, ACS Appl. Mater. Interfaces 5 (2013) 12168.

EP

[34] L. Ma, X. Shen, Z. Ji, S. Wang, H. Zhou, G. Zhu, Carbon coated nickel sulfide/reduced graphene oxide nanocomposites: facile synthesis and excellent supercapacitor

AC C

performance, Electrochim. Acta 146 (2014) 525. [35] K. Nielsch, J. Choi, K. Schwirn, Self-ordering regimes of porous alumina: the 10 porosity rule, Nano Letters, 2 (2002) 677. [36] H. Tamon, H. Ishizaka, T. Yamamoto, T. Suzuki, Preparation of mesoporous carbon by freeze drying, Carbon 37 (1999) 2049. [37] C. Li, X. Zhang, P. Yu, H. Zhang, X. Sun, Y. Ma, Soft template-assisted synthesis of single crystalline β-cobalt hydroxide with distinct morphologies, CrystEngComm 16 21

ACCEPTED MANUSCRIPT (2014) 7478. [38] X. Zang, Z. Dai, J. Yang, Y. Zhang, W. Huang, X. Dong, Template-sssisted synthesis of Nickel Sulfide nanowires: Tuning the compositions for supercapacitors with improved

RI PT

electrochemical stability, ACS Appl. Mater. Interfaces 8 (2016) 24645. [39] J. Archana, M. Navaneethan, Y. Hayakawa, S. Ponnusamy, C. Muthamizhchelvan, Effects of multiple organic ligands on size uniformity and optical properties of ZnSe

SC

quantum dots, Mater. Res. Bull. 47 (2012) 1892.

M AN U

[40] S. Harish, M. Navaneethan, J. Archana, A. Silambarasan, Y. Hayakawa, S. Ponnusamy, C. Muthamizhchelvan, Controlled synthesis of organic ligand passivated ZnO nanostructures and their photocatalytic activity under visible light irradiation, Dalton Trans. 44 (2015) 10490.

TE D

[41] W. Lina, L. Jiajia, L.Z. Li, D. Baosong, X. Meng, J. Muwei, X.S. Zhao, C. Chuanbao, Z. Jiatao, Z. Hesun, Rigid three-dimensional Ni3S4 nanosheet frames: controlled synthesis

EP

and their enhanced electrochemical performance, RSC Adv. 5 (2015) 8422.

AC C

[42] M. Navaneethan, K.D. Nisha, S. Ponnusamy, C. Muthamizhchelvan, Optical and surface morphological properties of triethylamine passivated lead sulphide nanoparticles, Mater. Chem. Phys. 117 (2009) 443. [43] M. Navaneethan, J. Archana, M. Arivandhan, Y. Hayakawa, Functional properties of amine-passivated ZnO nanostructures and dye-sensitized solar cell characteristics, Chem Eng J. 213 (2012) 70. 22

ACCEPTED MANUSCRIPT [44] Y. Zhang, L. Zuo, L. Zhang, J. Yan, H. Lu, W. Fan, T. Liu, Immobilization of NiS nanoparticles on N-doped carbon fiber aerogels as advanced electrode materials for supercapacitors, Nano Res. 9 (2016) 2747. [45] A.N. Naveen, S. Selladurai, Novel low temperature synthesis and electrochemical

RI PT

characterization of mesoporous nickel cobaltite-reduced graphene oxide (RGO) composite for supercapacitor application, Electrochim. Acta 173 (2015) 290.

electrodes, Electrochim. Acta 35 (1990) 263.

SC

[46] S. Ardizzone, G. Fregonara, S. Trasatti, “Inner” and “Outer” active surface of RuO2,

[47] N. Naveen, S. Kee-Sun, P. Myoungho, Graphite oxide as a Robust and efficient template

M AN U

for Co3O4-based high power battery-type electrodes, J. Electrochem. Soc. A214.

164 (2017)

[48] S.K. Meher, G.R. Rao, Ultralayered Co3O4 for high-performance supercapacitor applications, J. Phys. Chem. C 115 (2011) 15646.

TE D

[49] W. Jiang,F. Hu, Q. Yan, X. Wu, Investigation on electrochemical behaviors of NiCo2O4 battery-type supercapacitor electrodes:the role of an aqueous electrolyte, Inorg. Chem. Front. 4 (2017) 1642

EP

[50] B. Guan, Y. Li, B. Yin, K. Liu, D. Wang, H. Zhang, C. Cheng, Synthesis of hierarchical NiS microflowers for high performance asymmetric supercapacitor, Chem. Eng. J. 308

AC C

(2017) 1165.

[51] W. Jianghong, C. Ouyang, D. Shuo, W. Shuangyin, Hybrid NiS/CoO mesoporous nanosheets arrays on Ni foam for high-rate Supercapacitors, Nanotechnology 26 (2015) 325401. [52] L. Guo-Chang, L. Minmin, W. Meng-Ke, L. Peng-Fei, Z. Zhou, Z. Shuai-Ru, L. Rui, H. Lei, MOF-derived self-sacrificing route to hollow NiS2/ZnS nanospheres for high performance supercapacitors, RSC Adv. 6 (2016) 103517. 23

ACCEPTED MANUSCRIPT [53] N. Lengyuan, W. Yidan, R. Fengping, S. Cheng, S. Shen, X. Man, S. Zhenkun, L. Can, L. Xinjuan, G. Yinyan, In situ growth of NiCo2S4@Ni3V2O8 on Ni foam as a binder–free electrode for asymmetric supercapacitors, J. Mater. Chem. A 4 (2016) 5669. Figure caption

RI PT

Figure 1: X-ray diffraction pattern of NiS nanostructures.

Figure 2: (A) and (B) shows the XPS survey spectra and high resolution Ni 2p spectra of (a) NiS-P, (b) NiS-A, (c) NiS-B, (d) NiS-C and (e) NiS-D respectively.

SC

Figure 3: Displays the low and high magnification FESEM images of NiS-P ((A) and (B)), NiS-A ((C) and (D)), NiS-B ((E) and (F)), NiS-C ((G) and (H)) and NiS-D ((I) and (J).

M AN U

Figure 4: TEM and HR-TEM images of NiS-P ((A) and (B)) and NiS-C ((C) and (D)), respectively. Scale bar for TEM is 50 nm and HRTEM is 5 nm.

Figure 5: Schematic illustrating the growth mechanism of the hierarchical NiS microspheres formed by the self-assembly of HNPs.

TE D

Figure 6: Shows the CV curve recorded at 20 mV/s sweep rate in 3 M KOH electrolyte. Figure 7: Shows a plot of cathodic charge versus inverse of square root of scan rate for (A) NiS-P and (C) NiS-C. (B) and (D) graphically represents the quantity of charge stored via two

EP

kind of electrode process.

Figure 8: (A-E) shows the CD profile of NiS-P, NiS-A, NiS-B, NiS-C and NiS-D respectively

AC C

at various current densities like 0.5, 1, 2, 3, 4, 5, 10 and 20 A/g. (F) depicts the specific capacity retention at various current densities. Figure 9: (A) shows the results of 2000 continuous CD cycles at 10 A/g for NiS-P and NiS-C. (B) displays the EIS spectra recorded within a frequency range of 1 Hz to 500 kHz. Inset of the figure shows the magnified portion of the low frequency region and Randle’s equivalent circuit that fits the experimental data. Figure 10: (A) CV curve recorded at 20 mV/s for AC and NiS-C, (B) CV curve compared NiS-C/AC full-cell recorded at various scan rate (5, 10, 25, 50, 75 and 100 mV/s), (C) Galvanostatic charge-discharge 24

ACCEPTED MANUSCRIPT

AC C

EP

TE D

M AN U

SC

RI PT

curves at different current density (0.5, 1, 1.5, 2, 2.5, 5, 10 A/g) and (D) Ragone plot.

Fig. 1.

25

AC C

EP

TE D

M AN U

SC

RI PT

ACCEPTED MANUSCRIPT

Fig. 2.

26

AC C

EP

TE D

M AN U

SC

RI PT

ACCEPTED MANUSCRIPT

Fig. 3. 27

AC C

EP

TE D

M AN U

SC

RI PT

ACCEPTED MANUSCRIPT

Fig. 4.

28

AC C

EP

TE D

M AN U

SC

RI PT

ACCEPTED MANUSCRIPT

Fig. 5.

29

M AN U

SC

RI PT

ACCEPTED MANUSCRIPT

AC C

EP

TE D

Fig. 6.

Fig. 7. 30

Fig. 8.

AC C

EP

TE D

M AN U

SC

RI PT

ACCEPTED MANUSCRIPT

31

AC C

EP

TE D

M AN U

SC

RI PT

ACCEPTED MANUSCRIPT

Fig. 9.

32

M AN U

SC

RI PT

ACCEPTED MANUSCRIPT

AC C

EP

TE D

Fig. 10.

33

ACCEPTED MANUSCRIPT Table 1. Comparison Table S. No.

Composition

Max. SC

Potential window

Rate performance

Cycling result

Ref. No.

α-NiS hollow spheres

717.3 F/g @0.6 A/g

0.3 V

17%; 125.4 F/g @6 A/g

98.5%; @1.2 A/g for 1000 cycles

[21]

2

NiS2 nanocubes

695 F/g @1.25 A/g

0.4 V

22.7%; 158 F/g

93.5%; @1.25

[22]

3

Flower-like β-NiS

857.7 F/g @2 A/g

0.5 V

RI PT

1

@12.5 A/g

A/g for 3000 cycles

59%; 513 F/g

40%; @2 A/g for

@5 A/g

1000 cycles

62.8%; 583 F/g @10.2 A/g

52%; @4.2 A/g for 3000 cycles

[24]

65%; 503 F/g @ 5 A/g

88%; @2 A/g for 1000 cycles

[32]

[23]

NiS hollow spheres

927 F/g @4.2 A/g

0.7 V

5

NiS/Graphene nanosheets

775 F/g @0.5 A/g

0.4 V

6

NiS-Graphene Oxide

800 F/g @ 1A/g

1V

NA

98% @1 A/g for 1000 cycles

[33]

7

NiS-rGO composite

905 F/g @0.5 A/g

0.5 V

63%; 579 F/g @ 5 A/g

90%; @4A/g for 2000 cycles

[34]

8

Ni3S2-MWCNT

1024 F/g @ 0.8 A/g

0.45 V

46%; 480 F/g

80%; 3.2 A/g for

[35]

@25.6 A/g

1000 cycles

24%; 209 F/g @80 mV/s

98.6%; 5 A/g for 500 cycles

[36]

28%; 323.5 F/g 97.8%; 10 A/g for @30 A/g 1000 cycles

[50]

10

NiS microflowers

11

NiS/CoO nanosheets

M AN U

860 F/g @5 mV/s

0.5 V

1122.7 F/g @ 1 A/g

0.6 V

NiS2/ZnS nanospheres

1054 F/g @ 6 A/g

0.45 V

1198 F/g @ 1 A/g

0.6 V

AC C

12

TE D

Carbon coated Ni3S4-rGO

EP

9

SC

4

72%; 760 F/g

91.7%; 10 A/g for

@ 20 A/g

3000 cycles

49%; 592 F/g

87%; 5 A/g for

@ 10 A/g

1000 cycles

13

NiCo2S4/Ni3V2O8

853.3 F/g @ 1 A/g

0.6 V

77%; 660 F/g @ 10 A/g

94%; 5 A/g for 5000 cycles

14

This work

1212 F/g @0.5 A/g

0.5 V

50%; 604 F/g @20 A/g

93%; @10 A/g for 2000 cycles

34

[51] [52] [53]