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Ni(OH)2 nanocomposites for supercapacitor electrode
Accepted Manuscript Enhanced pseudocapacitance of NiSe2/Ni(OH)2 nanocomposites for supercapacitor electrode N. Sabari Arul, Jeong In Han PII: DOI: Reference:
S0167-577X(18)31440-X https://doi.org/10.1016/j.matlet.2018.09.064 MLBLUE 24932
To appear in:
Materials Letters
Received Date: Revised Date: Accepted Date:
30 May 2018 7 September 2018 12 September 2018
Please cite this article as: N. Sabari Arul, J. In Han, Enhanced pseudocapacitance of NiSe2/Ni(OH)2 nanocomposites for supercapacitor electrode, Materials Letters (2018), doi: https://doi.org/10.1016/j.matlet.2018.09.064
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.
Enhanced pseudocapacitance of NiSe2/Ni(OH)2 nanocomposites for supercapacitor electrode N. Sabari Arul* and Jeong In Han* Department of Chemical and Biochemical Engineering, Dongguk University-Seoul, 04620, Seoul, Republic of Korea. *Corresponding author E-mail: [email protected] [email protected]
Abstract We report a novel NiSe2/Ni(OH)2 nanocomposites (NNCs) synthesized using facile hydrothermal method followed by ultrasonication and employed it as electrode for supercapacitor. The structural and compositional analysis confirmed the presence of NiSe2 and Ni(OH)2 in the synthesized NNCs. The NNCs electrodes delivered an ultrahigh specific capacitance (2212 F g-1) than pristine NiSe2 (326 F g-1) at a current density of 2 mA cm-2 with capacitance retention of 95% after 5,000 cycles. The enhanced performance is due to the positive synergetic effect between NiSe2 and Ni(OH)2, which provides a free diffusion pathway for the fast ion transport and facile ion accessibility to storage sites. Keywords: Nanocomposites; Two dimensional dichalcogenide; NiSe2/Ni(OH)2; Supercapacitor; Energy storage and conversion.
1. Introduction Day by day the growth of advanced electronic devices has raised the demands of high-performance energy storage devices [1]. As an outcome, tremendous efforts have been devoted to expand new versatile and flexible electrode materials as substitutes to the materials used in existing batteries and supercapacitors [2]. Supercapacitor, also known as electrochemical capacitors, have been renowned as the most proficient energy storage device which has mesmerized considerable interest due to its high power density, excellent rate capability and long cycle life [3]. The electrochemical performance of the supercapacitor is mainly reliant upon the electrode material. Recently, transition metal compounds (ex. oxides, sulfides, hydroxides) have become spotlight due to its rich electrochemical faradic reaction but they suffer from poor conductivity which increases the sheet resistance as well as charge transfer resistance of the electrode resulting in the interior resistance loss at high current density [4]. Therefore, developing the electrode material with high electrical conductivity remains as the essential factor in obtaining the high performance supercapacitors. Nowadays, metal selenide based nanomaterials have been reported as excellent electrode material than metal sulfides for supercapacitor application due to its high electronic conductivity (ΩSe=1×10-3 S m-1 >
1
ΩS=1×10-28 S m-1) [5-9]. In particular, there are only few reports on nickel selenide (NiSe2) based electrode material for supercapacitor application [10-13]. Our group have reported two dimensional NiSe2 as electrode material for supercapacitor application with excellent cycling stability but with low specific capacitance [14]. Forming a binary composite with a metal oxide/hydroxide was proven to a suitable way to enhance the specific capacitance of the electrode [15]. Therefore, in order to enhance the specific capacitance of NiSe2, we have prepared NiSe2/Ni(OH)2 nanocomposites and studied its electrochemical performance. Nickel hydroxide (Ni(OH)2) is chosen as composite material because of its advantages such as high theoretical specific capacitance (2358 F g-1), excellent chemical stability and cost effectiveness [16]. To best of our knowledge, there is no report on the electrochemical performance of NiSe2/Ni(OH)2 nanocomposites (NNCs) as electrode material for supercapacitor application. In our present work, the prepared NNCs delivered an ultrahigh specific capacitance of 2212 F g-1 at 2 mA cm-2 with cycling stability of 95% for 5,000 cycles.
2. Experimental The NNCs was synthesized using facile hydrothermal method as follows, initially 0.5 M of Ni(CH3COO)2.4H2O and NaOH were mixed well in a beaker containing 60 ml of de-ionized water and ethanol which was stirred vigorously until the solution get a homogenous mixture. Then, the obtained solution was transferred into a stainless steel autoclave and maintained at 70 oC for 6 h. The obtained products were collected, washed with de-ionized water and dried in vacuum oven at 90 oC to obtain the β-Ni(OH)2 nanostructures. The hexapod-like NiSe2 microstructures was prepared using the procedure reported previously [11]. Finally, 1 mg of NiSe2 and β-Ni(OH)2 nanostructures were mixed well via ultrasonic agitation for about 30 min in order to obtain NNCs. The crystal structure and phase purity of the obtained products were determined by Rigaku Ultima IV diffractometer using CuKα radiation (λ=0.154 nm, 40 KV) at a scanning rate of 4 °/min. X-ray photoelectron microscopy (XPS) was investigated using a Theta Probe AR-XPS System from Thermo Fisher Scientific with monochromated Al Kα (1486.6 eV) as an X-ray source operating at 15 kV. The nitrogen adsorption/desorption isotherm and pore size analysis were carried out using Autosorb-Quantachrome instrument. The scanning electron microscopic (SEM) images were captured with FESEM JEOL F610F-Plus at an accelerating voltage of 15 kV. The transmission electron microscopy (TEM) images were captured using JEM-JEOL 2000EX instrument. The electrochemical measurement was performed on Biologic SP-150 electrochemical workstation using three electrode system in 3M KOH electrolyte. The nickel foam (1×1 cm2) were used as working
2
electrode, a platinum wire as counter electrode, and Hg/HgO as reference electrode, respectively. The active material (1 mg of NiSe2 and NNCs) was coated on the working electrode using carbon black, polyvinylidene fluoride with NMP in a weight ratio of 80:15:5 and dried at 50 ºC for overnight.
3. Results and Discussion Fig. 1a shows the X-ray diffraction pattern of synthesized pristine NiSe2 and NNCs. The pristine NiSe2 and NNCs showed diffraction peaks along (200), (210), (211), (220), (221), (311), (222), (023), (321) and (400) planes that can be assigned to NiSe2 (JCPDS card no. 65-1843) [17]. The NNCs exhibits few additional peaks along (100), (101), (110) and (111) planes which correspond to the β-Ni (OH)2 (JCPDS no.14-0117) [18]. In addition, the surface compositions of NNCs were carried out using XPS analysis. Fig. 1b shows the high-resolution XPS spectrum of Ni 2p which can be deconvoluted into six peaks with binding energy of Ni 2p3/2 (855.8 eV) and Ni 2p1/2 (873.3 eV). The results are in good agreement with previoulsy reported NiSe2 [11, 19]. Fig. 1c displays the core level spectrum of O 1s which can be deconvoluted into three peaks located at 529.8 eV, 530.8 eV and 532.2 eV that can be attributed to Ni-O-H, O-H and Ni-O-Ni respectively [20]. The excess hydroxyl groups present on the Ni surface of NNCs can increase the wettability between electrode and electrolytes are beneficial for the enhancement of the electrochemical properties [21]. The binding energy spectrum of Se 3d (Fig. 1d) shows a prominent peak S1 at 54.9 eV and a weak peak S2 at 59.8 eV that corresponds to Se-O bonding at the surface [22, 23]. Fig.1e shows the nitrogen adsorption-desorption isotherm of NNCs that exhibited type IV isotherm curve with apparent hysteresis loop measured in the range of 0.4-1.0 P/Po, indicating the presence of mesopores in the material. The specific surface area of NNCs is found to be ~89 m2 g-1. In addition, the pore size distribution (Fig. 1f) was calculated using Barrett-Joyner-Halenda analysis and found that the synthesized NNCs have mesopores with average diameter of 3.837 nm. Thus, the achieved high specific surface area plays a major role to enhance the electroactive sites which is advantageous for the improvement of electron transport kinetics. In addition, the mesopores of NNCs electrode provides multiple channels for the ion/electron transport kinetics resulting in the enhanced electrochemical properties [24]. Fig. 2a-e displays the FESEM images of hexapod-like NiSe2 microstructures composed of nanoparticles with size less than ~10 nm. Fig. 2f-i shows the FESEM images of NNCs. In addition, insight structural analysis of NNCs was investigated using TEM images shown in Fig. 2j-l. The inset of Fig. 2j shows the NiSe2 hexapod-like microstructures [14]. It is obvious from Fig. 2j that NNCs is composed of NiSe2 and β-
3
Ni(OH)2. In addition, high resolution TEM images in Fig. 2k & l shows the lattice spacing of 0.275 nm and 0.24 nm, which matches well with (210) and (101) planes of NiSe2 and β-Ni(OH)2 respectively. The electrochemical performance of pristine NiSe2 and NNCs is characterized using cyclic voltammetric (CV) analysis. Fig. 3a shows the comparative CV curves of bare nickel substrate, pristine NiSe2 and NNCs at a scan rate of 20 mV s-1. In addition, the CV curves of pristine NiSe2 and NNCs at various scan rates is provided in the Fig.3b,c which exhibits symmetric redox peaks corresponds to the redox reaction which can be written as follows, NiSe2 + H2O+1/2O2 → Ni(OH)2 + 2Se and Ni(OH)2 + OH- ↔ NiOOH + H2O + e[11]. Fig. 3d and e shows the galvanostatic charge discharge (GCD) curves of pristine NiSe2 and NNCs at various current densities that revealed the symmetric and non-linear behaviour of the electrode materials. The estimated high Csp of pristine NiSe2 and NNCs shown in Fig. 3f was found to be 326 F g-1 and 2212 F g-1. The ultrahigh Csp of NNCs is due to the formation of NiSe2/Ni(OH)2 composite structure which provides a lot of interspaces between NiSe2 and Ni (OH)2 facilitating the diffusion of the electrolyte [25]. Table 1 shows the comparison of Csp of the obtained NNCs with reported nickel selenide and sulfides in the literature [6, 10, 11, 26-29]. Moreover, charge transfer kinetics of pristine NiSe2 and NNCs electrode was investigated using electrochemical impedance spectroscopy (EIS) analysis. Fig.3g shows the EIS spectra of pristine NiSe2 and NNCs. The equivalent series resistance (ESR) measured from the intercept of semi-circle on real axis is lower for NNCs (0.62 Ω) than pristine NiSe2 (0.7 Ω) which is beneficial for fast electronic/ionic transportation resulting in the enhanced electrochemical performance of the NNCs. Furthermore, the charge transfer resistance determined from the diameter of the semi-circle is smaller for NNCs (0.23 Ω) than pristine NiSe2 (1.12 Ω) which ultimately leads to enhanced charge transfer ability of NNCs. In addition, long term stability is a crucial factor in supercapacitor application and we have investigated the cycling stability of pristine NiSe2 and NNCs at a high current density of 20 mA cm-2 for successive 5,000 cycles (Fig. 3h). It is clear that, NNCs electrode exhibited excellent cycling stability of 95% after 5000 cycles demonstrating its better stability than pristine NiSe2 (67%). Besides, it is obvious that NNCs electrode exhibited high coulombic efficiency (Fig. 3i) signifying enhanced ionic diffusion resulting in the improved electrochemical performance and demonstrating its potential use as electrode material for supercapacitor application.
4
4. Conclusion We have synthesized NNCs using facile hydrothermal method followed by ultrasonication and further investigated as electrode material in supercapacitor. The electrochemical analysis showed that the NNCs electrode exhibited an ultrahigh specific capacitance of 2212 F g-1 at a current density of 2 mA cm-2 with a cycling stability of 95% for 5,000 cycles. Our results showed that the NNCs electrode could be a potential candidate for next generation supercapacitor electrode materials for energy storage applications.
Acknowledgment This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (NRF-2017R1D1A1B03030456).
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Figure caption Fig.1 (a) XRD spectra of a NiSe2 and NiSe2/β-Ni(OH)2 nanocomposites, XPS core-level spectrum of (b) Ni 2p (c) O 1s and (d) Se 3d of NiSe2/β-Ni(OH)2 nanocomposites (e) Nitrogen adsorption/desorption isotherms (f) Pore-size distribution pattern of NiSe2/β-Ni(OH)2 nanocomposites. Fig.2 FESEM images of (a-e) NiSe2 (f-i) NiSe2/β-Ni(OH)2 nanocomposites (j) TEM images of NiSe2/β-Ni(OH)2 nanocomposites (Inset of (j) shows the pristine NiSe2 hexapod-like microstructures [14]) (k-l) High-resolution TEM images of NiSe2/β-Ni(OH)2 nanocomposites. Fig.3 (a) Comparative CV curves of bare nickel foam, NiSe2 and NiSe2/β-Ni(OH)2 nanocomposites (b) CV curves of NiSe2 (c) CV curves of NiSe2/β-Ni(OH)2 nanocomposites (d) GCD curves of NiSe2 (e) GCD curves of NiSe2/β-Ni(OH)2 nanocomposites (f) Specific capacitance of NiSe2/β-Ni(OH)2 nanocomposites at various current densities (g) EIS spectra of NiSe2 and NiSe2/β-Ni(OH)2 nanocomposites [Inset shows the enlarged EIS spectra] (h) Cycling stability of NiSe2 and NiSe2/β-Ni(OH)2 nanocomposites (i) Coulombic efficiency of pristine NiSe2 and NiSe2/β-Ni(OH)2 nanocomposites electrodes.
Fig.1
50
(400), (111)
(023) (321) (110)
(311) (222)
(220)
40
(221)
(101)
30
XPS Intensity (arb. units)
(210), (100)
NNCs
(211) (200)
XRD Intensity (arb. units)
Prisitine NiSe2
Ni 2p 3/2
860.09 eV 862.5 eV
850
60
S2 (59.8 eV)
60
Binding energy (eV)
66
Volume adsorbed (cm 3 g-1)
XPS Intensity (arb. units)
873.3 eV 876.82 eV 880.6 eV
870
880
OH (530.8 eV)
526
20
0.2
528
530
532
534
(f)
NNCs
40
0.0
(532.2 eV)
Binding energy (eV)
60
0
Ni-O-Ni
Ni-O-H (529.8 eV)
890
Pore volume (cm 3 g-1 nm -1)
(e)
Se 3d
54
860
O 1s
Binding energy (eV)
S1 (54.9 eV )
48
Ni 2p 1/2
855.8 eV
2 (degree)
(d)
(c)
Ni 2p
XPS Intensity (arb. units)
(b)
(a)
0.4
0.6
0.8
1.0
Relative Pressure (P/Po)
6
1.2
0.09
0.06
0.03
0.00 0
10
20
Pore diameter (nm)
30
40
Fig. 2
(a) NiSe2
10 μm
(d)
100 nm
(g)
(b)
(c)
1 μm
1 μm
(e)
(f) NNCs
100 nm
1 μm
(h)
(i)
Ni (OH)2
NiSe2 1 μm
(j)
100 nm
100 nm
(k)
(l) 0.275 nm, (210)
0.5 μm
10 nm
0.24 nm, (101) 10 nm
7
Fig. 3 (b) Pristine NiSe
(a) 45 30
0
-10
15 0 -15
0.1
0.2
0.3
0.4
0.5
0.1
0.2
0.4
0.0
0.5
0.1
Potential (V) vs Hg/HgO
0.4 0.3 -2
2 mA cm -2 3 mA cm -2 5 mA cm -2 7 mA cm -2 9 mA cm
0.2 0.1 0.0 100
200
0.3 -2
2 mA cm -2 3 mA cm -2 5 mA cm -2 7 mA cm -2 9 mA cm
0.2 0.1 0.0 0
200
400
600
800
Pristine NiSe2 NNCs
30
1
2
Z' (Ohm)
20 10
Pristine NiSe2 NNCs
0 10
20
30
NNCs
2000 1500 1000 500 0
1000 1200 1400 1600
1
40
50
60
4
5
6
7
8
9
10
(i)
80 60 Pristine NiSe2 NNCs
20 0 0
3
-2
100
40
2
Current density (mA cm )
Coulombic Efficiency (%)
0
Specific capacitance (F g-1)
-Z" (Ohm)
40
1
0.5
NiSe2
2500
(h)
50
0.4
3000
Time (s)
(g)
0.3
(f)
0.4
300
0.2
Potential (V) vs Hg/HgO
0.5
Time (s)
-Z" (Ohm)
0.3
(e) NNCs
(d) Pristine NiSe2
0
-15
Potential (V) vs Hg/HgO
0.5
60
0
-30
0.0
Potential (V) vs Hg/HgO
0
15
Specific capacitance (F g-1)
0.0
Potential (V) vs Hg/HgO
30
-30
-20
-1
10 mV s -1 20 mV s -1 30 mV s -1 40 mV s -1 50 mV s -1 70 mV s -1 100 mV s
1000
2000
Z' (Ohm)
3000
4000
5000
100 104
Pristine NiSe2 NNCs
90
100
80 96 0
1000
2000
3000
4000
5000
Cycle number
Cycle number
Table 1. Comparison of Csp of NiSe2/β-Ni(OH)2 nanocomposites with reported nickel selenide and sulfides.
Material NiSe2 hollow sphere
Specific capacitance 341 F g-1 at 1 A g-1
NiSe2/carbon fiber cloth
1058 F g-1 at 2 A g-1 -1
-1
Truncated cube like NiSe2 single crystals NiS nanostructures NiS microflowers
1044 F g at 3 A g
α-NiS hollow spheres
717.3 F g-1 at 0.6 A g-1
NiSe microspheres
492 F g-1 at 0.5 A g-1
NiSe2/Ni(OH)2 nanocomposites
2212 F g-1 at 2 A g-1
964 F g-1 at 1 A g-1 1122.7 F g-1 at 1 A g-1
8
Cycling stability 269 F g-1 after 1000 cycles 82% after 2000 cycles 67% after 2000 cycles 97.8% after 1000 cycles 1.5% loss after 1000 cycles 84.6% after 200 cycles 95% after 5000 cycles
Ref. [6] [10] [11] [26] [27] [28] [29] This work
Coulombic Efficiency (%)
NNCs
10
45
Current (mA)
Bare nickel foam Pristine NiSe2
Current (mA)
Current (mA)
20
(c) NNCs
2
-1
10 mV s -1 20 mV s -1 30 mV s -1 40 mV s -1 50 mV s -1 70 mV s -1 100 mV s
Highlights
NiSe2/Ni(OH)2 nanocomposites (NNCs) for high performance supercapacitor electrode.
NNCs delivered a high specific capacitance of 2212 F g-1 than pristine NiSe2 (326 F g1
).
NNCs showed excellent capacitance retention of 95% for 5,000 cycles.
9
Graphical Abstract
10
S0167-577X(18)31440-X https://doi.org/10.1016/j.matlet.2018.09.064 MLBLUE 24932
To appear in:
Materials Letters
Received Date: Revised Date: Accepted Date:
30 May 2018 7 September 2018 12 September 2018
Please cite this article as: N. Sabari Arul, J. In Han, Enhanced pseudocapacitance of NiSe2/Ni(OH)2 nanocomposites for supercapacitor electrode, Materials Letters (2018), doi: https://doi.org/10.1016/j.matlet.2018.09.064
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.
Enhanced pseudocapacitance of NiSe2/Ni(OH)2 nanocomposites for supercapacitor electrode N. Sabari Arul* and Jeong In Han* Department of Chemical and Biochemical Engineering, Dongguk University-Seoul, 04620, Seoul, Republic of Korea. *Corresponding author E-mail: [email protected] [email protected]
Abstract We report a novel NiSe2/Ni(OH)2 nanocomposites (NNCs) synthesized using facile hydrothermal method followed by ultrasonication and employed it as electrode for supercapacitor. The structural and compositional analysis confirmed the presence of NiSe2 and Ni(OH)2 in the synthesized NNCs. The NNCs electrodes delivered an ultrahigh specific capacitance (2212 F g-1) than pristine NiSe2 (326 F g-1) at a current density of 2 mA cm-2 with capacitance retention of 95% after 5,000 cycles. The enhanced performance is due to the positive synergetic effect between NiSe2 and Ni(OH)2, which provides a free diffusion pathway for the fast ion transport and facile ion accessibility to storage sites. Keywords: Nanocomposites; Two dimensional dichalcogenide; NiSe2/Ni(OH)2; Supercapacitor; Energy storage and conversion.
1. Introduction Day by day the growth of advanced electronic devices has raised the demands of high-performance energy storage devices [1]. As an outcome, tremendous efforts have been devoted to expand new versatile and flexible electrode materials as substitutes to the materials used in existing batteries and supercapacitors [2]. Supercapacitor, also known as electrochemical capacitors, have been renowned as the most proficient energy storage device which has mesmerized considerable interest due to its high power density, excellent rate capability and long cycle life [3]. The electrochemical performance of the supercapacitor is mainly reliant upon the electrode material. Recently, transition metal compounds (ex. oxides, sulfides, hydroxides) have become spotlight due to its rich electrochemical faradic reaction but they suffer from poor conductivity which increases the sheet resistance as well as charge transfer resistance of the electrode resulting in the interior resistance loss at high current density [4]. Therefore, developing the electrode material with high electrical conductivity remains as the essential factor in obtaining the high performance supercapacitors. Nowadays, metal selenide based nanomaterials have been reported as excellent electrode material than metal sulfides for supercapacitor application due to its high electronic conductivity (ΩSe=1×10-3 S m-1 >
1
ΩS=1×10-28 S m-1) [5-9]. In particular, there are only few reports on nickel selenide (NiSe2) based electrode material for supercapacitor application [10-13]. Our group have reported two dimensional NiSe2 as electrode material for supercapacitor application with excellent cycling stability but with low specific capacitance [14]. Forming a binary composite with a metal oxide/hydroxide was proven to a suitable way to enhance the specific capacitance of the electrode [15]. Therefore, in order to enhance the specific capacitance of NiSe2, we have prepared NiSe2/Ni(OH)2 nanocomposites and studied its electrochemical performance. Nickel hydroxide (Ni(OH)2) is chosen as composite material because of its advantages such as high theoretical specific capacitance (2358 F g-1), excellent chemical stability and cost effectiveness [16]. To best of our knowledge, there is no report on the electrochemical performance of NiSe2/Ni(OH)2 nanocomposites (NNCs) as electrode material for supercapacitor application. In our present work, the prepared NNCs delivered an ultrahigh specific capacitance of 2212 F g-1 at 2 mA cm-2 with cycling stability of 95% for 5,000 cycles.
2. Experimental The NNCs was synthesized using facile hydrothermal method as follows, initially 0.5 M of Ni(CH3COO)2.4H2O and NaOH were mixed well in a beaker containing 60 ml of de-ionized water and ethanol which was stirred vigorously until the solution get a homogenous mixture. Then, the obtained solution was transferred into a stainless steel autoclave and maintained at 70 oC for 6 h. The obtained products were collected, washed with de-ionized water and dried in vacuum oven at 90 oC to obtain the β-Ni(OH)2 nanostructures. The hexapod-like NiSe2 microstructures was prepared using the procedure reported previously [11]. Finally, 1 mg of NiSe2 and β-Ni(OH)2 nanostructures were mixed well via ultrasonic agitation for about 30 min in order to obtain NNCs. The crystal structure and phase purity of the obtained products were determined by Rigaku Ultima IV diffractometer using CuKα radiation (λ=0.154 nm, 40 KV) at a scanning rate of 4 °/min. X-ray photoelectron microscopy (XPS) was investigated using a Theta Probe AR-XPS System from Thermo Fisher Scientific with monochromated Al Kα (1486.6 eV) as an X-ray source operating at 15 kV. The nitrogen adsorption/desorption isotherm and pore size analysis were carried out using Autosorb-Quantachrome instrument. The scanning electron microscopic (SEM) images were captured with FESEM JEOL F610F-Plus at an accelerating voltage of 15 kV. The transmission electron microscopy (TEM) images were captured using JEM-JEOL 2000EX instrument. The electrochemical measurement was performed on Biologic SP-150 electrochemical workstation using three electrode system in 3M KOH electrolyte. The nickel foam (1×1 cm2) were used as working
2
electrode, a platinum wire as counter electrode, and Hg/HgO as reference electrode, respectively. The active material (1 mg of NiSe2 and NNCs) was coated on the working electrode using carbon black, polyvinylidene fluoride with NMP in a weight ratio of 80:15:5 and dried at 50 ºC for overnight.
3. Results and Discussion Fig. 1a shows the X-ray diffraction pattern of synthesized pristine NiSe2 and NNCs. The pristine NiSe2 and NNCs showed diffraction peaks along (200), (210), (211), (220), (221), (311), (222), (023), (321) and (400) planes that can be assigned to NiSe2 (JCPDS card no. 65-1843) [17]. The NNCs exhibits few additional peaks along (100), (101), (110) and (111) planes which correspond to the β-Ni (OH)2 (JCPDS no.14-0117) [18]. In addition, the surface compositions of NNCs were carried out using XPS analysis. Fig. 1b shows the high-resolution XPS spectrum of Ni 2p which can be deconvoluted into six peaks with binding energy of Ni 2p3/2 (855.8 eV) and Ni 2p1/2 (873.3 eV). The results are in good agreement with previoulsy reported NiSe2 [11, 19]. Fig. 1c displays the core level spectrum of O 1s which can be deconvoluted into three peaks located at 529.8 eV, 530.8 eV and 532.2 eV that can be attributed to Ni-O-H, O-H and Ni-O-Ni respectively [20]. The excess hydroxyl groups present on the Ni surface of NNCs can increase the wettability between electrode and electrolytes are beneficial for the enhancement of the electrochemical properties [21]. The binding energy spectrum of Se 3d (Fig. 1d) shows a prominent peak S1 at 54.9 eV and a weak peak S2 at 59.8 eV that corresponds to Se-O bonding at the surface [22, 23]. Fig.1e shows the nitrogen adsorption-desorption isotherm of NNCs that exhibited type IV isotherm curve with apparent hysteresis loop measured in the range of 0.4-1.0 P/Po, indicating the presence of mesopores in the material. The specific surface area of NNCs is found to be ~89 m2 g-1. In addition, the pore size distribution (Fig. 1f) was calculated using Barrett-Joyner-Halenda analysis and found that the synthesized NNCs have mesopores with average diameter of 3.837 nm. Thus, the achieved high specific surface area plays a major role to enhance the electroactive sites which is advantageous for the improvement of electron transport kinetics. In addition, the mesopores of NNCs electrode provides multiple channels for the ion/electron transport kinetics resulting in the enhanced electrochemical properties [24]. Fig. 2a-e displays the FESEM images of hexapod-like NiSe2 microstructures composed of nanoparticles with size less than ~10 nm. Fig. 2f-i shows the FESEM images of NNCs. In addition, insight structural analysis of NNCs was investigated using TEM images shown in Fig. 2j-l. The inset of Fig. 2j shows the NiSe2 hexapod-like microstructures [14]. It is obvious from Fig. 2j that NNCs is composed of NiSe2 and β-
3
Ni(OH)2. In addition, high resolution TEM images in Fig. 2k & l shows the lattice spacing of 0.275 nm and 0.24 nm, which matches well with (210) and (101) planes of NiSe2 and β-Ni(OH)2 respectively. The electrochemical performance of pristine NiSe2 and NNCs is characterized using cyclic voltammetric (CV) analysis. Fig. 3a shows the comparative CV curves of bare nickel substrate, pristine NiSe2 and NNCs at a scan rate of 20 mV s-1. In addition, the CV curves of pristine NiSe2 and NNCs at various scan rates is provided in the Fig.3b,c which exhibits symmetric redox peaks corresponds to the redox reaction which can be written as follows, NiSe2 + H2O+1/2O2 → Ni(OH)2 + 2Se and Ni(OH)2 + OH- ↔ NiOOH + H2O + e[11]. Fig. 3d and e shows the galvanostatic charge discharge (GCD) curves of pristine NiSe2 and NNCs at various current densities that revealed the symmetric and non-linear behaviour of the electrode materials. The estimated high Csp of pristine NiSe2 and NNCs shown in Fig. 3f was found to be 326 F g-1 and 2212 F g-1. The ultrahigh Csp of NNCs is due to the formation of NiSe2/Ni(OH)2 composite structure which provides a lot of interspaces between NiSe2 and Ni (OH)2 facilitating the diffusion of the electrolyte [25]. Table 1 shows the comparison of Csp of the obtained NNCs with reported nickel selenide and sulfides in the literature [6, 10, 11, 26-29]. Moreover, charge transfer kinetics of pristine NiSe2 and NNCs electrode was investigated using electrochemical impedance spectroscopy (EIS) analysis. Fig.3g shows the EIS spectra of pristine NiSe2 and NNCs. The equivalent series resistance (ESR) measured from the intercept of semi-circle on real axis is lower for NNCs (0.62 Ω) than pristine NiSe2 (0.7 Ω) which is beneficial for fast electronic/ionic transportation resulting in the enhanced electrochemical performance of the NNCs. Furthermore, the charge transfer resistance determined from the diameter of the semi-circle is smaller for NNCs (0.23 Ω) than pristine NiSe2 (1.12 Ω) which ultimately leads to enhanced charge transfer ability of NNCs. In addition, long term stability is a crucial factor in supercapacitor application and we have investigated the cycling stability of pristine NiSe2 and NNCs at a high current density of 20 mA cm-2 for successive 5,000 cycles (Fig. 3h). It is clear that, NNCs electrode exhibited excellent cycling stability of 95% after 5000 cycles demonstrating its better stability than pristine NiSe2 (67%). Besides, it is obvious that NNCs electrode exhibited high coulombic efficiency (Fig. 3i) signifying enhanced ionic diffusion resulting in the improved electrochemical performance and demonstrating its potential use as electrode material for supercapacitor application.
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4. Conclusion We have synthesized NNCs using facile hydrothermal method followed by ultrasonication and further investigated as electrode material in supercapacitor. The electrochemical analysis showed that the NNCs electrode exhibited an ultrahigh specific capacitance of 2212 F g-1 at a current density of 2 mA cm-2 with a cycling stability of 95% for 5,000 cycles. Our results showed that the NNCs electrode could be a potential candidate for next generation supercapacitor electrode materials for energy storage applications.
Acknowledgment This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (NRF-2017R1D1A1B03030456).
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Figure caption Fig.1 (a) XRD spectra of a NiSe2 and NiSe2/β-Ni(OH)2 nanocomposites, XPS core-level spectrum of (b) Ni 2p (c) O 1s and (d) Se 3d of NiSe2/β-Ni(OH)2 nanocomposites (e) Nitrogen adsorption/desorption isotherms (f) Pore-size distribution pattern of NiSe2/β-Ni(OH)2 nanocomposites. Fig.2 FESEM images of (a-e) NiSe2 (f-i) NiSe2/β-Ni(OH)2 nanocomposites (j) TEM images of NiSe2/β-Ni(OH)2 nanocomposites (Inset of (j) shows the pristine NiSe2 hexapod-like microstructures [14]) (k-l) High-resolution TEM images of NiSe2/β-Ni(OH)2 nanocomposites. Fig.3 (a) Comparative CV curves of bare nickel foam, NiSe2 and NiSe2/β-Ni(OH)2 nanocomposites (b) CV curves of NiSe2 (c) CV curves of NiSe2/β-Ni(OH)2 nanocomposites (d) GCD curves of NiSe2 (e) GCD curves of NiSe2/β-Ni(OH)2 nanocomposites (f) Specific capacitance of NiSe2/β-Ni(OH)2 nanocomposites at various current densities (g) EIS spectra of NiSe2 and NiSe2/β-Ni(OH)2 nanocomposites [Inset shows the enlarged EIS spectra] (h) Cycling stability of NiSe2 and NiSe2/β-Ni(OH)2 nanocomposites (i) Coulombic efficiency of pristine NiSe2 and NiSe2/β-Ni(OH)2 nanocomposites electrodes.
Fig.1
50
(400), (111)
(023) (321) (110)
(311) (222)
(220)
40
(221)
(101)
30
XPS Intensity (arb. units)
(210), (100)
NNCs
(211) (200)
XRD Intensity (arb. units)
Prisitine NiSe2
Ni 2p 3/2
860.09 eV 862.5 eV
850
60
S2 (59.8 eV)
60
Binding energy (eV)
66
Volume adsorbed (cm 3 g-1)
XPS Intensity (arb. units)
873.3 eV 876.82 eV 880.6 eV
870
880
OH (530.8 eV)
526
20
0.2
528
530
532
534
(f)
NNCs
40
0.0
(532.2 eV)
Binding energy (eV)
60
0
Ni-O-Ni
Ni-O-H (529.8 eV)
890
Pore volume (cm 3 g-1 nm -1)
(e)
Se 3d
54
860
O 1s
Binding energy (eV)
S1 (54.9 eV )
48
Ni 2p 1/2
855.8 eV
2 (degree)
(d)
(c)
Ni 2p
XPS Intensity (arb. units)
(b)
(a)
0.4
0.6
0.8
1.0
Relative Pressure (P/Po)
6
1.2
0.09
0.06
0.03
0.00 0
10
20
Pore diameter (nm)
30
40
Fig. 2
(a) NiSe2
10 μm
(d)
100 nm
(g)
(b)
(c)
1 μm
1 μm
(e)
(f) NNCs
100 nm
1 μm
(h)
(i)
Ni (OH)2
NiSe2 1 μm
(j)
100 nm
100 nm
(k)
(l) 0.275 nm, (210)
0.5 μm
10 nm
0.24 nm, (101) 10 nm
7
Fig. 3 (b) Pristine NiSe
(a) 45 30
0
-10
15 0 -15
0.1
0.2
0.3
0.4
0.5
0.1
0.2
0.4
0.0
0.5
0.1
Potential (V) vs Hg/HgO
0.4 0.3 -2
2 mA cm -2 3 mA cm -2 5 mA cm -2 7 mA cm -2 9 mA cm
0.2 0.1 0.0 100
200
0.3 -2
2 mA cm -2 3 mA cm -2 5 mA cm -2 7 mA cm -2 9 mA cm
0.2 0.1 0.0 0
200
400
600
800
Pristine NiSe2 NNCs
30
1
2
Z' (Ohm)
20 10
Pristine NiSe2 NNCs
0 10
20
30
NNCs
2000 1500 1000 500 0
1000 1200 1400 1600
1
40
50
60
4
5
6
7
8
9
10
(i)
80 60 Pristine NiSe2 NNCs
20 0 0
3
-2
100
40
2
Current density (mA cm )
Coulombic Efficiency (%)
0
Specific capacitance (F g-1)
-Z" (Ohm)
40
1
0.5
NiSe2
2500
(h)
50
0.4
3000
Time (s)
(g)
0.3
(f)
0.4
300
0.2
Potential (V) vs Hg/HgO
0.5
Time (s)
-Z" (Ohm)
0.3
(e) NNCs
(d) Pristine NiSe2
0
-15
Potential (V) vs Hg/HgO
0.5
60
0
-30
0.0
Potential (V) vs Hg/HgO
0
15
Specific capacitance (F g-1)
0.0
Potential (V) vs Hg/HgO
30
-30
-20
-1
10 mV s -1 20 mV s -1 30 mV s -1 40 mV s -1 50 mV s -1 70 mV s -1 100 mV s
1000
2000
Z' (Ohm)
3000
4000
5000
100 104
Pristine NiSe2 NNCs
90
100
80 96 0
1000
2000
3000
4000
5000
Cycle number
Cycle number
Table 1. Comparison of Csp of NiSe2/β-Ni(OH)2 nanocomposites with reported nickel selenide and sulfides.
Material NiSe2 hollow sphere
Specific capacitance 341 F g-1 at 1 A g-1
NiSe2/carbon fiber cloth
1058 F g-1 at 2 A g-1 -1
-1
Truncated cube like NiSe2 single crystals NiS nanostructures NiS microflowers
1044 F g at 3 A g
α-NiS hollow spheres
717.3 F g-1 at 0.6 A g-1
NiSe microspheres
492 F g-1 at 0.5 A g-1
NiSe2/Ni(OH)2 nanocomposites
2212 F g-1 at 2 A g-1
964 F g-1 at 1 A g-1 1122.7 F g-1 at 1 A g-1
8
Cycling stability 269 F g-1 after 1000 cycles 82% after 2000 cycles 67% after 2000 cycles 97.8% after 1000 cycles 1.5% loss after 1000 cycles 84.6% after 200 cycles 95% after 5000 cycles
Ref. [6] [10] [11] [26] [27] [28] [29] This work
Coulombic Efficiency (%)
NNCs
10
45
Current (mA)
Bare nickel foam Pristine NiSe2
Current (mA)
Current (mA)
20
(c) NNCs
2
-1
10 mV s -1 20 mV s -1 30 mV s -1 40 mV s -1 50 mV s -1 70 mV s -1 100 mV s
Highlights
NiSe2/Ni(OH)2 nanocomposites (NNCs) for high performance supercapacitor electrode.
NNCs delivered a high specific capacitance of 2212 F g-1 than pristine NiSe2 (326 F g1
).
NNCs showed excellent capacitance retention of 95% for 5,000 cycles.
9
Graphical Abstract
10