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Rational design of NiCo2S4 nanoparticles @ N-doped CNT for hybrid supercapacitor
Accepted Manuscript Full Length Article Rational design of NiCo2S4 nanoparticles @ N-doped CNT for hybrid supercapacitor Yuting Luan, Henan Zhang, Fan Yang, Jun Yan, Kai Zhu, Ke Ye, Guiling Wang, Kui Cheng, Dianxue Cao PII: DOI: Reference:
S0169-4332(18)30933-4 https://doi.org/10.1016/j.apsusc.2018.03.236 APSUSC 38984
To appear in:
Applied Surface Science
Received Date: Revised Date: Accepted Date:
2 January 2018 23 March 2018 29 March 2018
Please cite this article as: Y. Luan, H. Zhang, F. Yang, J. Yan, K. Zhu, K. Ye, G. Wang, K. Cheng, D. Cao, Rational design of NiCo2S4 nanoparticles @ N-doped CNT for hybrid supercapacitor, Applied Surface Science (2018), doi: https://doi.org/10.1016/j.apsusc.2018.03.236
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Rational design of NiCo2S4 nanoparticles @ N-doped CNT for hybrid supercapacitor Yuting Luana, Henan Zhanga, Fan, Yangb*, Jun Yana, Kai Zhua, Ke Yea, Guiling Wanga, Kui Chenga*, Dianxue Caoa a. Key Laboratory of Superlight Material and Surface Technology of Ministry of Education, College of Material Science and Chemical Engineering, Harbin Engineering University, Harbin, China b. College of Science, Northeast Agricultural University, Harbin 150030, China Abstract A simple hydrothermal route is designed to decorate NiCo 2S4 nanoparticles on the surface of N-doped carbon nanotubes to form a coaxial composite (NiCo2S4@NCNT). Inherited the high electrical conductivity from the NCNT and high capacitive performance of NiCo2S4, the optimized NiCo2S4@NCNT composite could significantly reduce the contact resistance and effectively increase the transfer rate of ion and electron and thus benefit for its electrochemical performance enhancement. When employed as a battery-type supercapacitor electrode, the NiCo2S4@NCNT composite exhibits a high capacitance up to 783.5 C g-1 at 1 A g-1 and as well as rate performance (74.6 % retention with the current density increases from 1 to 50 A g-1). Coupled with activated carbon (AC) negative electrode, the as-assembled hydride supercapacitor delivers a maximum energy density of 49.75 Wh kg-1 at a power density of 774.65 W kg-1, as well as 88.9 % capacitance retained after 3000 cycles at a
Corresponding author. E-mail address: [email protected] (Dr. Kui Cheng), [email protected] (Dr. Fan Yang) [email protected] (Dr. Dianxue Cao). 1
current density of 10 A g-1. These above results demonstrate the enormous potential of NiCo2S4@NCNT in the development of hybrid supercapacitors. Key Words Hybrid supercapacitor; Energy density; CNT; NiCo2S4; Core-shell. Introduction Supercapacitor has been considered as an attractive energy storage device to bridge the gap between the battery and electric capacitor due to its relatively high energy density than electric capacitor and high-power density than batteries.[1-3] Nevertheless, the restriction of energy density hinders its widespread application.[4] According to the energy density equation E=1/2CV2, designing a hybrid supercapacitor (HSC) that employing a battery-type electrode to deliver high energy density and an electric double layer capacitor electrode to provide high power density is an ideal way to solve the above issue.[5] In general, the intrinsic charge storage performance of the electrode materials, especially the battery-type electrode, plays an important role on the overall performance of HSC. Various transition metal oxide/hydroxide such as MnO2, NiO, Ni(OH)2 and NiCo2O4,[6-19] have been significant made for the electrode of HSC based on their lower price and lower toxic nature. Unfortunately, the inferior electrical conductivity of these materials is unfavorable for fast electron transport required at higher power output. Recently, nickel-cobalt sulfide has been considered as a typical battery-type electrode material for HSC due to its outstanding reversible redox behavior, and higher electric conductivity (about 10 times) than that of their counterparts 2
(Nickel-cobalt oxides).[20] However, the practicable application of pure nickel-cobalt sulfides without special structure has been restricted by its relative low specific surface area, agglomeration and large volume alter in the course of charge and discharge processes along with other battery-types capacitor materials, leading to needy rate capability and cycling stability. [21-22] An effective strategy to overcome these problems is to conveniently regulate the morphology and structure of nickel-cobalt sulfides (e. g. nanotube,[23] nanourchin,[24] nanosheets,[25] nanocubic,[26] and nanoflower,[27] etc.) and further immobilite these nanomaterials on carbonaceous materials with rich porosity, high SSA and high conductivity (activated carbon, mesoporous carbon, graphene and carbon nanotubes etc.).[28] For example, G. B. Lemu et al. reported that by combining the nickel-cobalt sulfide core-shell structure with 3D graphene, the resulting composite exhibits a higher areal capacitance of 15.6 F cm-2 at 10 mA cm-2, and an excellent cycling stability of 93 % after 5000 at a current density of 10 mA cm-2.[29] D. L. Li et al. have shown that the CNTs/NiCo 2S4 hybrid electrode exhibited an excellent battery-like capacitive performance, a high capacitance up to 768 C g-1 (discharge current density of 1 A g-1) and an outstanding rate capability of 78.1 % retention at the density of 100 A g-1.[30] Fu et al. reported that hierarchical NiCo2S4 nanowires coated with polyaniline exhibited remarkable electrochemical performances 4.74 F cm-2 at 5 mA cm-2 and rate capability of 47.67 % at current density of 50 mA cm-2.[31] Niu and his co-workers prepared hierarchical NiCo2S4@Ni3V2O8 compound using a hydrothermal process combined with co-precipitation method and the results showed that as-prepared product possessed a 3
high specific capacitance of 512 C g -1 at 1 A g-1.[32] Jae-Jin Shim and his co-worker prepared NiCo2S4 and NiCo2O4 compound also shown high electrochemical performance.[33-36] Despite the progress to date, further enhancement of the capacitance performance of NiCo2S4 is necessary for their practical application in HSC. It is known to all that the nitrogen located at the edges of graphene layers, such as pyrrolic nitrogen (N-5) and pyridinic nitrogen (N-6), are considered to introduce a pseudocapacitance effect, whereas quaternary-type nitrogen (N-Q) carrying positive charges can promote electron transfer through the carbon. Moreover, the incorporation of N atoms into the carbon materials can change the structure of adjacent carbon atoms and provide a large number of surface defects, and as well as increase electrode wettability, thus improving the capacitive performance of carbon electrodes.[20] Hence, in this paper, we report a facile two-step route to immobilite NiCo2S4 nanoparticles on the surface of N-doped carbon nanotube (NCNT). The NiCo2O4 nanoparticles were first grow on the NCNT through an electrostatic self-assembly reaction and the subsequent hydrothermal process would get the end-product NiCo2S4@NCNT.
Benefiting
from
the
unique
architecture
structure,
the
NiCo2S4@NCNT hybrid offers an enhanced electrochemical performance, that the capacitance performance can reach 783.5 C g-1 at 1 A g-1 with long cycle stability. The HSC assembled with NiCo2S4@NCNT as positive electrode and AC negative electrode delivers a high energy density of 49.75 Wh kg-1, as well as excellent cycling stability. The results suggesting the nickel-cobalt sulphur hybrid material is a promising material for HSC application. 4
2. Experimental section 2.1. Material synthesis Prior to use, the purchased CNT (Shenzhen Nnaotech Port Co., Ltd., China) was treated according to the previous report.[37] The NiCo2S4@NCNT composite was synthesized through an electrostatic self-assembly reaction and a subsequent hydrothermal process. In briefly, 50 mg CNT was ultrasonically dispersed in the mixture solvent of 58 mL ethanol and 12 mL deionized water to form a uniformly suspension ink, and then, 0.2 mmol Co(AC)2·4H2O and 0.1 mmol Ni(AC)2·4H2O were added and further stirred for 2 h under 90 °C to form the Ni-Co precursor. Then the 0.9 mmol thiourea and 1 mL ammonium hydroxide were following added in the mixture solution. After continuous stirring for 1 h, the mixture was transferred into a 50-mL autoclave and heated at 160 °C for 5 h. After the autoclave was cooled to room temperature, the solid products were centrifugal washed and dried in vacuum oven overnight. For comparisons, NiCo2S4@CNT was prepared following similar procedures in the absence of ammonium hydroxide. NiCo2S4 was prepared following similar procedures in the absence of CNT and ammonium hydroxide. 2.2. Characterization The micro-nanostructured morphologies of the as-synthesized samples were respectively characterized by Scanning electron microscopy (SEM) (Hitachi S-4800) and Transmission electron microscopy (TEM, JEOL JEM-2100). The structure were Raman spectra were performed using a confocal Raman microscope (DXR, Thermo-Fisher Scientific) at 532 nm excitation from an argon ion laser. Powder X-ray 5
diffraction (Rigaku TTR III with Cu Ka radiation) was used to characterize the structure of samples. X-ray photoelectron spectroscopy (XPS) analysis was performed on a Thermo ESCALAB 250. Thermogravimetric analysis (TGA) was carried out with a Netzsch STA 449C thermal analyzer. 2.3. Electrochemical measurements The Electrochemical measurements were first evaluated in a conventional three-electrode electrochemical cell equipped with a platinum foil as the counter electrode and saturated calomel electrode (SCE) reference electrode and the electrolyte is 6 M KOH aqueous electrolyte. The electrode was prepared by mixing the as-prepared samples with poly (tetrafluoroethylene) (PTFE) and acetylene black at a weight ratio of 8:1:1 in an agate mortar to form a homogeneous black slurry and then coated on a 1×1 cm2 nickel foam current collector with a 4 mg mass loading, and finally dried at 80 °C overnight. The cyclic voltammetrys (CVs), galvanostatic charging-discharging (GCD) and electrochemical impedance spectroscopy (EIS) were performed on computerized potentiostat (Autolab PGSTAT302, Eco Chemie) controlled by GPES software. The working voltage windows were set from 0 to 0.5 V. The Specific capacity (C) (for the NiCo2S4@NCNT, NiCo2S4@CNT and NiCo2S4 electrodes) or specific capacitance (Cs) (for the AC electrode) of the three-electrode system was calculated from the GCD curves using the following equation:
C
I t m
Cs
I t mV
(1)
(2)
6
where Δt is the discharge time (s), I is the discharge current (A), m is the total mass of active material in the electrode (g), and ΔV is the potential range for the charge/discharge process (V). An HSC was assembled using NiCo2S4@NCNT as the positive electrode and AC negative electrode in a 6 M KOH electrolyte solution. In the HSC device, the NiCo2S4@NCNT//AC electrode was considered as the reference electrode. To balance the charge storage in the positive and negative electrodes, the charge storage in the positive electrode (Q+) and negative electrode (Q-) should follow the equation:
Q Q _
(3)
Q C m
(4)
Q _ Cs _ V m _
(5)
Mass balance is expressed as:
m Cs V m C
(6)
The specific capacity (C’) of HSC was calculated from the GCD curves as follows:
C'
I t m ' V
(7)
where m’ is the total mass of electroactive materials in the positive and negative electrodes (g). The energy density E (Wh kg -1) and power densities P (W kg -1) of the HSC were calculated as follows:
1 E C 'V 2 2
(8)
E t
(9)
P
7
where Δt is the discharge time and V (V) is the cell voltage excluding the IR drop. 3. Results and discussion
Figure 1. The SEM (a), TEM (b), HRTEM (c), EDS spectrum (d), the typical HAADF-STEM images with corresponding EDS element mappings (e), the inset of (b) is the corresponding histograms of the particle size distributions.
The microscopic morphology of the NiCo 2S4@NCNT hybrid was explored by SEM and TEM. As shown in Figure 1a, the NiCo2S4@NCNT hybrid shows the NiCo2S4 nanoparticles coated on the surface of NCNT to form an evenly three-dimensional network-like nanostructure and the morphology of NiCo 2S4@NCNT hybrid is in keeping with that of NiCo2S4@CNT (Figure S1). The SEM images of pure NiCo2S4 and CNT were shown in the Figure S2a and 2b, respectively. Without the additional CNT, the NiCo2S4 nanoparticles were inevitably conglomerated. The TEM image distinctly confirms the NiCo2S4 nanoparticles well dispersed on the NCNT with a near logarithmic normal distribution and the mean particle diameter was about 25 nm. (Figure 1b). Moreover, the high-resolution TEM (HR-TEM) image (Figure 1c) reveals 8
a continuous lattice fringes indicated high crystallization feature of the obtained NiCo2S4 nanoparticles. In additional, the spaces of the neighbor lattice fringes are about 0.29 and 0.34 nm, which are consistent with the (311) and (220) lattice spacing of NiCo2S4, respectively. The high-angle annular dark-field scanning TEM (HAADF-STEM) image with corresponding EDS mappings are shown in Figure 1e, unambiguously confirms the evenly distributed throughout of the C, N, Ni, Co, and S elements and the atomic ratio of Ni: Co: S is about 1:2:4 (Figure 1d).
Figure 2. XRD patterns of (a) NiCo2S4, NiCo2S4@CNT, and NiCo2S4@NCNT; Raman spectra of (b) CNT, NiCo2S4@CNT, and NiCo2S4@NCNT, N2 adsorption-desorption isotherms (c) and pore-size distribution curves (d) of the NiCo2S4@NCNT, NiCo2S4@CNT and NiCo2S4.
The structure of NiCo2S4@NCNT was characterized by X-ray diffraction (XRD) and the results are shown in Figure 2a. The typical diffraction peaks appeared at 2θ of 26.5°, 31.7°, 38.1°, 50.3°, and 55.1° are assigned to the (220), (331), (400), (511), and (440) crystal planes of NiCo2S4 (JCPDS No. 20-0782), respectively.[38] It is noted that 9
the peak intensities of the NiCo 2S4 are increased with the introduction of CNT and/or NCNT, which may result from the CNT could provide more active sites to support the NiCo2S4 grow. However, the diffraction peaks of CNT are not observed probably because the surface of CNT were coated with NiCo2S4 particles. Therefore, Raman spectrum was performed to confirm the existence of CNT and the results are presented in Figure 2b. Evidently, two characteristic peaks of the D (1350 cm-1) and G band (1590 cm-1) originates from the CNT were identified in the hybrid of NiCo2S4@NCNT and the relative strength (ID/IG) between the D and G bands corresponds to the degree of graphitization. [39] The ID/IG value of NiCo2S4@NCNT is about 0.91 which was higher than that of NiCo 2S4@CNT (about 0.79) and CNT (about 0.45). This phenomenon indicates the defect rich characteristic caused by the N doping in the sp2 carbon structure, which is due to the use of ammonium hydroxide during the hydrothermal process.[40-41] The N2 adsorption-desorption test was performed to evaluate the specific surface area of the NiCo2S4@NCNT. As shown in the Figure 2c, the specific surface areas of the NiCo2S4@NCNT, NiCo2S4@CNT and NiCo2S4 were calculated to be 75.34, 73.47 and 35.12 m2 g-1, respectively. The pore-size distribution structures were further revealed by applying the BJH method to the adsorption branch of the isotherm (Figure 2d). The peaks located at 2.35 and 45.83 nm reveal the co-exist of meso-pore and marc-pore. Such textural characteristics could provide rich electro-active sites for the energy storage and short diffusion paths for ion and electron transports.
10
Figure 3. The full-scan (a) and the high-resolution Ni 2p (b), Co 2p (c), S 2p (d), N 1s (e) and C 1s (f) XPS spectra of NiCo2S4@NCNT.
The surface species and chemical states of NiCo2S4@NCNT were explored further by XPS measurements. The survey spectrum of the NiCo2S4@NCNT confirms the presence of C 1s, N 1s, O 1s, Ni 2p, Co 2p and S 2p peaks (Figure 3a), consisting with the results of XRD and Raman. The high-resolution XPS spectrum of Ni 2p can be mainly deconvoluted into two spin-orbit doublets with shakeup satellites (marked as “Sat.”), ascribing to Ni 2p3/2 (855.6 eV) and Ni 2p1/2 (873.1 eV), respectively (Figure 3b), suggesting the coexistence of Ni2+ and Ni3+.[42] Meanwhile, there is predominant Co 2p3/2 and Co 2p1/2 peaks centered at 780.8 and 795.9 eV with a spin-orbit splitting of 15.1 eV, suggesting the coexistence of Co 2+/Co3+ species (Figure 3c).[43-44] Moreover, S 2p characteristic photoelectron peaks were found to appear at 163.8 (S 2p1/2) and 162.0 eV (S 2p3/2), respectively (Figure 3d). The peak of S 2p1/2 is typical of a metal-sulphur bond, and the peak of S 2p3/2 is attributed to the sulfur ions (S2-) in a low coordination which on the surface of the hybrid. [45] These results suggest that the 11
Ni, Co, S match well with the reported data of NiCo 2S4 and the NiCo2S4 nanoparticles is assembled onto the NCNT. In addition, the pyridinic N (398.6 eV), pyrrolic N (399.8 eV), graphitic N (400.9 eV) and C-N (285.7 eV) are clearly shown at the high-resolution N and C 1s XPS spectra of NiCo2S4@NCNT (Figure 3e and 3f),[46-48] which suggests that the N are successfully introduced into the CNT and more active sites are provided on the surface of NiCo2S4@NCNT.
Figure 4. The CV (a) and GCD (b) curves of NiCo2S4, NiCo2S4@CNT and NiCo2S4@NCNT at 20 mV s-1 and a current density of 1 A g-1, respectively; The CV (c) and GCD (d) curves of NiCo2S4@NCNT at different scan rates and different current densities; The Specific capacity (e) and Nyquist plots (f) of NiCo2S4, NiCo2S4@CNT and NiCo2S4@NCNT NiCo2S4; The capacitance retention (g) of NiCo2S4@NCNT at a current density of 20 A g-1 after 5000 cycle numbers; (h) The part of charge and discharge curves during the cycling progress.
To explore the superiority of electrochemical performances of the samples, the CV and GCD measurements were first performed in three-system electrode equipped with 6 M KOH solution as electrolyte, the compared CV curves of NiCo2S4@NCNT, NiCo2S4@CNT and NiCo2S4 recorded at a scan rate of 20 mV s-1 from 0 to 0.5 V are shown in Figure 4a. Obviously, the NiCo2S4@NCNT exhibited the most promising 12
capacitance performance in terms of its largest integrated area, indicating the robust faradaic behavior as battery-type electrode materials. Besides, the CV curves show a pair
redox peak,
corresponding
to
the transformation of
the
following
equations:[20,23,49] NiS + OH- NiSOH +e
( 10 )
CoS+OH- CoSOH +e
(11)
CoSOH+OH- CoSO+H2O+e
(12)
The GCD curves measured at a current density of 1 A g -1 were conducted to further evaluate the battery-type properties of the as-synthesized samples. It can be clearly observed that the NiCo 2S4-CNT@N electrode displays the longest charge-discharge time, demonstrated the superior battery-type performance, further proved that doping nitrogen (N) species into carbon materials is another strategy of improving energy storage ability of electrode materials. Meanwhile, the GCD curves displayed a pair distinct plateau region, precisely agree with the results of CV curves, which further verified the battery-type feature. The CVs of NiCo2S4@NCNT record at various scan rates are performed in Figure 4c. With the scan rates increased from 5 mV s-1 to 100 mV s-1, the anode peak moves towards high potential and the cathode peak moves towards low potential at the same time, which is attributed to the polarization effect of electrode. In addition, the peak current densities linearly increase with the scan rates and the redox shapes remind at high scan rates, indicating the rapid redox reaction of NiCo2S4@NCNT.[50] The GCD measurements were also performed at different current densities to further evaluate the electrochemical performance of the 13
as-prepared N-doped electrodes, and the results are shown in Figure 4d. The specific capacity was calculated according to the Eq. (1) and the results are summarized in Figure 4e. Apparently, the specific capacity of NiCo2S4@NCNT, NiCo2S4@CNT and NiCo2S4 are 783.5, 667, and 519 C g-1 at 1 A g-1, respectively, and decrease to be 584.5, 404.5, and 250.5 C g-1 at relativity high current density of 50 A g-1. In this respect, the capacity of NiCo2S4@NCNT electrode is much higher than the remaining electrodes and as well as the literature data summarized in Table S1 in the support information due to N dopant could enhance the electrical conductivity of CNT that act as conducting tunnel for charge transfer between CNT and NiCo2S4 and that is enhance the synergistic effect between them. The Figure 4f shows the EIS measurement performed at a frequency range from 0.01 Hz to 100 K Hz, and the inset is an expanded view at the high-frequency region. NiCo2S4@NCNT electrode show a very low equivalent series resistance, the value of the Rct about (0.16 Ω) as compared to NiCo2S4@CNT (0.21 Ω), and NiCo2S4 (0.32 Ω) (Figure S4), indicating that the NiCo2S4@NCNT sample have a small resistance and fast ion response at high-frequency region which is attributed to N dopant. Meanwhile, the NiCo2S4@NCNT sample has a more ideal straight line at high-frequency region, indicating a lower diffusion resistance.[51] Figure 4g show the long cyclic life test of the NiCo2S4@NCNT which was monitored by using galvanostatic charge-discharge measurement at a current density of 20 A g-1. After 5000 cycles, the hybrid NiCo2S4@NCNT electrode retains 86.46 % of the initial capacitance (Figure 4g), thus showing the outstanding structural stability. The part of charge and discharge progress 14
was shown in Figure 4h. The present work provides relativity higher specific capacitance and better rate capability even compared with other reported literatures (Table S1). In addition, the GCD and CV of NiCo 2S4@CNT and NiCo2S4 were tested with different current densities and scan rates which shown in the Figure S5a-5d, the specific capacity of NiCo 2S4@CNT and NiCo2S4 are 667 and 519 C g-1 at 1 A g-1, respectively, and decrease to be 404.5, and 250.5 C g-1 with current density of 50 A g-1. Moreover, NiCo2S4@CNT and NiCo2S4 was characterized by using galvanostatic charge-discharge measurement up to 5000 cycles at a scan rate 10 A g-1 as shown in Figure S5e. The capacitance decreased with the increasing of cycle number and maintained about 76.6 and 53.4% from the original after 5000 cycles.
Figure. 5 The CV (a) and GCD (b) curves of NiCo2S4@NCNT//AC asymmetric supercapacitor collected in different scan rate and different current density. (c) Energy density and power density of the asymmetric supercapacitor NiCo2S4@NCNT//AC; (d) The capacitance retention of NiCo2S4@NCNT//AC at a current density of 10 A g-1. 15
For further exploration the practical application, we used the NiCo2S4@NCNT electrode as positive electrode and AC as the negative electrode to fabricate an HSC. Activated carbon is an ideal supercapacitor electrode due to good conductivity and excellent electrochemical stability. The mass loading of AC and NiCo2S4@NCNT was according to the principle of charge balance. It is well known that the AC electrode behaved as an ideal EDLC with a potential window from -1.0 to 0 V, combined with the potential of NiCo2S4@NCNT from 0 to 0.5 V, the working voltage window increased to 1.5 V. Figure 5a show the CV curves of the HSC recorded at various scan rates. The shape of CV curves is nearly rectangular with a redox peak, demonstrating that the device exhibited nearly ideal battery-like electrochemical capacitive behavior. The superior electrochemical performance of the device was further confirmed by galvanostatic charge-discharge measurement and the values of specific capacity are calculated to be 226.7, 218, 199.5, and 189 C g-1 at the current densities 1, 2, 5, and 10 A g-1, respectively. When the current density come to 20 A g-1, a high specific capacity value of 177.8 C g-1 still could be obtained. Moreover, the rate capability with each current density was shown in the Figure 5b, which also indicates the remarkable capability of the device for high-rate energy storage applications. The energy
density
and
power
density
of
asymmetric
supercapacitor
NiCo2S4@NCNT//AC were calculated from galvanostatic discharge curves of Figure 5c. Notably, the device achieved a maximum energy density of 49.75 Wh kg -1 at a power density of 774.65 W kg-1 which is much higher than those of the latest reported NiCo2S4-based asymmetric and/or symmetric supercapacitors.[25,52-57] This high 16
energy density can be attributing to the active nanoparticles of NiCo 2S4 well assembled on the surface of N doped carbon nanotube and the good match of NiCo 2S4 with
the
N-doped
NiCo2S4@NCNT//AC
CNT. device
Furthermore, was
the
cycling
characterized
by
performance using
of
the
galvanostatic
charge-discharge measurement up to 3000 cycles at a scan rate 10 A g-1 as shown in Figure 5d. The capacitance decreased with the increasing of cycle number and maintained about 89.8 % from the original after 3000 cycles. Conclusions In this paper, a novel electrostatic self-assembly combined one-step hydrothermal method is performed to prepare NiCo2S4@NCNT hybrid. Benefit from its unique structure, that is the high special surface area of CNTs will shorten the diffusion path of ion and N dopant could validly reduce the electron transportation resistance, when employ NiCo2S4@NCNT hybrid as a battery-type electrode to assemble HSC, it demonstrated an outstanding electrochemical performance with a high energy density of 49.75 Wh kg-1 at a power density of 774.65 W kg-1. The device also possessed amazing cycle stability, therefore, the as-prepared HSC device is a promising candidate for high-performance energy storage devices in reality. ACKNOWLEDGMENT We gratefully acknowledge the financial support of this research by National Nature Science Foundation of China (21503055), the Hong Kong Scholars Programs (Grant No. XJ2016046), the China Postdoctoral Science Foundation (2015M571390), the Natural Science Foundation of Heilongjiang Province of China (QC2015015) and the 17
Heilongjiang Postdoctoral Fund (LBHZ14054, LBH-TZ0609).
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25
Highlight
NiCo2O4 nanoparticles are uniformly distributed on the surface of N-doped CNTs
The NiCo2S4@NCNT electrode exhibits high specific capacitances and cycle stability.
NiCo2O4@NCNT//AC hybrid supercapacitor exhibits high energy desity of 49.75 Wh kg-1.
26
S0169-4332(18)30933-4 https://doi.org/10.1016/j.apsusc.2018.03.236 APSUSC 38984
To appear in:
Applied Surface Science
Received Date: Revised Date: Accepted Date:
2 January 2018 23 March 2018 29 March 2018
Please cite this article as: Y. Luan, H. Zhang, F. Yang, J. Yan, K. Zhu, K. Ye, G. Wang, K. Cheng, D. Cao, Rational design of NiCo2S4 nanoparticles @ N-doped CNT for hybrid supercapacitor, Applied Surface Science (2018), doi: https://doi.org/10.1016/j.apsusc.2018.03.236
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.
Rational design of NiCo2S4 nanoparticles @ N-doped CNT for hybrid supercapacitor Yuting Luana, Henan Zhanga, Fan, Yangb*, Jun Yana, Kai Zhua, Ke Yea, Guiling Wanga, Kui Chenga*, Dianxue Caoa a. Key Laboratory of Superlight Material and Surface Technology of Ministry of Education, College of Material Science and Chemical Engineering, Harbin Engineering University, Harbin, China b. College of Science, Northeast Agricultural University, Harbin 150030, China Abstract A simple hydrothermal route is designed to decorate NiCo 2S4 nanoparticles on the surface of N-doped carbon nanotubes to form a coaxial composite (NiCo2S4@NCNT). Inherited the high electrical conductivity from the NCNT and high capacitive performance of NiCo2S4, the optimized NiCo2S4@NCNT composite could significantly reduce the contact resistance and effectively increase the transfer rate of ion and electron and thus benefit for its electrochemical performance enhancement. When employed as a battery-type supercapacitor electrode, the NiCo2S4@NCNT composite exhibits a high capacitance up to 783.5 C g-1 at 1 A g-1 and as well as rate performance (74.6 % retention with the current density increases from 1 to 50 A g-1). Coupled with activated carbon (AC) negative electrode, the as-assembled hydride supercapacitor delivers a maximum energy density of 49.75 Wh kg-1 at a power density of 774.65 W kg-1, as well as 88.9 % capacitance retained after 3000 cycles at a
Corresponding author. E-mail address: [email protected] (Dr. Kui Cheng), [email protected] (Dr. Fan Yang) [email protected] (Dr. Dianxue Cao). 1
current density of 10 A g-1. These above results demonstrate the enormous potential of NiCo2S4@NCNT in the development of hybrid supercapacitors. Key Words Hybrid supercapacitor; Energy density; CNT; NiCo2S4; Core-shell. Introduction Supercapacitor has been considered as an attractive energy storage device to bridge the gap between the battery and electric capacitor due to its relatively high energy density than electric capacitor and high-power density than batteries.[1-3] Nevertheless, the restriction of energy density hinders its widespread application.[4] According to the energy density equation E=1/2CV2, designing a hybrid supercapacitor (HSC) that employing a battery-type electrode to deliver high energy density and an electric double layer capacitor electrode to provide high power density is an ideal way to solve the above issue.[5] In general, the intrinsic charge storage performance of the electrode materials, especially the battery-type electrode, plays an important role on the overall performance of HSC. Various transition metal oxide/hydroxide such as MnO2, NiO, Ni(OH)2 and NiCo2O4,[6-19] have been significant made for the electrode of HSC based on their lower price and lower toxic nature. Unfortunately, the inferior electrical conductivity of these materials is unfavorable for fast electron transport required at higher power output. Recently, nickel-cobalt sulfide has been considered as a typical battery-type electrode material for HSC due to its outstanding reversible redox behavior, and higher electric conductivity (about 10 times) than that of their counterparts 2
(Nickel-cobalt oxides).[20] However, the practicable application of pure nickel-cobalt sulfides without special structure has been restricted by its relative low specific surface area, agglomeration and large volume alter in the course of charge and discharge processes along with other battery-types capacitor materials, leading to needy rate capability and cycling stability. [21-22] An effective strategy to overcome these problems is to conveniently regulate the morphology and structure of nickel-cobalt sulfides (e. g. nanotube,[23] nanourchin,[24] nanosheets,[25] nanocubic,[26] and nanoflower,[27] etc.) and further immobilite these nanomaterials on carbonaceous materials with rich porosity, high SSA and high conductivity (activated carbon, mesoporous carbon, graphene and carbon nanotubes etc.).[28] For example, G. B. Lemu et al. reported that by combining the nickel-cobalt sulfide core-shell structure with 3D graphene, the resulting composite exhibits a higher areal capacitance of 15.6 F cm-2 at 10 mA cm-2, and an excellent cycling stability of 93 % after 5000 at a current density of 10 mA cm-2.[29] D. L. Li et al. have shown that the CNTs/NiCo 2S4 hybrid electrode exhibited an excellent battery-like capacitive performance, a high capacitance up to 768 C g-1 (discharge current density of 1 A g-1) and an outstanding rate capability of 78.1 % retention at the density of 100 A g-1.[30] Fu et al. reported that hierarchical NiCo2S4 nanowires coated with polyaniline exhibited remarkable electrochemical performances 4.74 F cm-2 at 5 mA cm-2 and rate capability of 47.67 % at current density of 50 mA cm-2.[31] Niu and his co-workers prepared hierarchical NiCo2S4@Ni3V2O8 compound using a hydrothermal process combined with co-precipitation method and the results showed that as-prepared product possessed a 3
high specific capacitance of 512 C g -1 at 1 A g-1.[32] Jae-Jin Shim and his co-worker prepared NiCo2S4 and NiCo2O4 compound also shown high electrochemical performance.[33-36] Despite the progress to date, further enhancement of the capacitance performance of NiCo2S4 is necessary for their practical application in HSC. It is known to all that the nitrogen located at the edges of graphene layers, such as pyrrolic nitrogen (N-5) and pyridinic nitrogen (N-6), are considered to introduce a pseudocapacitance effect, whereas quaternary-type nitrogen (N-Q) carrying positive charges can promote electron transfer through the carbon. Moreover, the incorporation of N atoms into the carbon materials can change the structure of adjacent carbon atoms and provide a large number of surface defects, and as well as increase electrode wettability, thus improving the capacitive performance of carbon electrodes.[20] Hence, in this paper, we report a facile two-step route to immobilite NiCo2S4 nanoparticles on the surface of N-doped carbon nanotube (NCNT). The NiCo2O4 nanoparticles were first grow on the NCNT through an electrostatic self-assembly reaction and the subsequent hydrothermal process would get the end-product NiCo2S4@NCNT.
Benefiting
from
the
unique
architecture
structure,
the
NiCo2S4@NCNT hybrid offers an enhanced electrochemical performance, that the capacitance performance can reach 783.5 C g-1 at 1 A g-1 with long cycle stability. The HSC assembled with NiCo2S4@NCNT as positive electrode and AC negative electrode delivers a high energy density of 49.75 Wh kg-1, as well as excellent cycling stability. The results suggesting the nickel-cobalt sulphur hybrid material is a promising material for HSC application. 4
2. Experimental section 2.1. Material synthesis Prior to use, the purchased CNT (Shenzhen Nnaotech Port Co., Ltd., China) was treated according to the previous report.[37] The NiCo2S4@NCNT composite was synthesized through an electrostatic self-assembly reaction and a subsequent hydrothermal process. In briefly, 50 mg CNT was ultrasonically dispersed in the mixture solvent of 58 mL ethanol and 12 mL deionized water to form a uniformly suspension ink, and then, 0.2 mmol Co(AC)2·4H2O and 0.1 mmol Ni(AC)2·4H2O were added and further stirred for 2 h under 90 °C to form the Ni-Co precursor. Then the 0.9 mmol thiourea and 1 mL ammonium hydroxide were following added in the mixture solution. After continuous stirring for 1 h, the mixture was transferred into a 50-mL autoclave and heated at 160 °C for 5 h. After the autoclave was cooled to room temperature, the solid products were centrifugal washed and dried in vacuum oven overnight. For comparisons, NiCo2S4@CNT was prepared following similar procedures in the absence of ammonium hydroxide. NiCo2S4 was prepared following similar procedures in the absence of CNT and ammonium hydroxide. 2.2. Characterization The micro-nanostructured morphologies of the as-synthesized samples were respectively characterized by Scanning electron microscopy (SEM) (Hitachi S-4800) and Transmission electron microscopy (TEM, JEOL JEM-2100). The structure were Raman spectra were performed using a confocal Raman microscope (DXR, Thermo-Fisher Scientific) at 532 nm excitation from an argon ion laser. Powder X-ray 5
diffraction (Rigaku TTR III with Cu Ka radiation) was used to characterize the structure of samples. X-ray photoelectron spectroscopy (XPS) analysis was performed on a Thermo ESCALAB 250. Thermogravimetric analysis (TGA) was carried out with a Netzsch STA 449C thermal analyzer. 2.3. Electrochemical measurements The Electrochemical measurements were first evaluated in a conventional three-electrode electrochemical cell equipped with a platinum foil as the counter electrode and saturated calomel electrode (SCE) reference electrode and the electrolyte is 6 M KOH aqueous electrolyte. The electrode was prepared by mixing the as-prepared samples with poly (tetrafluoroethylene) (PTFE) and acetylene black at a weight ratio of 8:1:1 in an agate mortar to form a homogeneous black slurry and then coated on a 1×1 cm2 nickel foam current collector with a 4 mg mass loading, and finally dried at 80 °C overnight. The cyclic voltammetrys (CVs), galvanostatic charging-discharging (GCD) and electrochemical impedance spectroscopy (EIS) were performed on computerized potentiostat (Autolab PGSTAT302, Eco Chemie) controlled by GPES software. The working voltage windows were set from 0 to 0.5 V. The Specific capacity (C) (for the NiCo2S4@NCNT, NiCo2S4@CNT and NiCo2S4 electrodes) or specific capacitance (Cs) (for the AC electrode) of the three-electrode system was calculated from the GCD curves using the following equation:
C
I t m
Cs
I t mV
(1)
(2)
6
where Δt is the discharge time (s), I is the discharge current (A), m is the total mass of active material in the electrode (g), and ΔV is the potential range for the charge/discharge process (V). An HSC was assembled using NiCo2S4@NCNT as the positive electrode and AC negative electrode in a 6 M KOH electrolyte solution. In the HSC device, the NiCo2S4@NCNT//AC electrode was considered as the reference electrode. To balance the charge storage in the positive and negative electrodes, the charge storage in the positive electrode (Q+) and negative electrode (Q-) should follow the equation:
Q Q _
(3)
Q C m
(4)
Q _ Cs _ V m _
(5)
Mass balance is expressed as:
m Cs V m C
(6)
The specific capacity (C’) of HSC was calculated from the GCD curves as follows:
C'
I t m ' V
(7)
where m’ is the total mass of electroactive materials in the positive and negative electrodes (g). The energy density E (Wh kg -1) and power densities P (W kg -1) of the HSC were calculated as follows:
1 E C 'V 2 2
(8)
E t
(9)
P
7
where Δt is the discharge time and V (V) is the cell voltage excluding the IR drop. 3. Results and discussion
Figure 1. The SEM (a), TEM (b), HRTEM (c), EDS spectrum (d), the typical HAADF-STEM images with corresponding EDS element mappings (e), the inset of (b) is the corresponding histograms of the particle size distributions.
The microscopic morphology of the NiCo 2S4@NCNT hybrid was explored by SEM and TEM. As shown in Figure 1a, the NiCo2S4@NCNT hybrid shows the NiCo2S4 nanoparticles coated on the surface of NCNT to form an evenly three-dimensional network-like nanostructure and the morphology of NiCo 2S4@NCNT hybrid is in keeping with that of NiCo2S4@CNT (Figure S1). The SEM images of pure NiCo2S4 and CNT were shown in the Figure S2a and 2b, respectively. Without the additional CNT, the NiCo2S4 nanoparticles were inevitably conglomerated. The TEM image distinctly confirms the NiCo2S4 nanoparticles well dispersed on the NCNT with a near logarithmic normal distribution and the mean particle diameter was about 25 nm. (Figure 1b). Moreover, the high-resolution TEM (HR-TEM) image (Figure 1c) reveals 8
a continuous lattice fringes indicated high crystallization feature of the obtained NiCo2S4 nanoparticles. In additional, the spaces of the neighbor lattice fringes are about 0.29 and 0.34 nm, which are consistent with the (311) and (220) lattice spacing of NiCo2S4, respectively. The high-angle annular dark-field scanning TEM (HAADF-STEM) image with corresponding EDS mappings are shown in Figure 1e, unambiguously confirms the evenly distributed throughout of the C, N, Ni, Co, and S elements and the atomic ratio of Ni: Co: S is about 1:2:4 (Figure 1d).
Figure 2. XRD patterns of (a) NiCo2S4, NiCo2S4@CNT, and NiCo2S4@NCNT; Raman spectra of (b) CNT, NiCo2S4@CNT, and NiCo2S4@NCNT, N2 adsorption-desorption isotherms (c) and pore-size distribution curves (d) of the NiCo2S4@NCNT, NiCo2S4@CNT and NiCo2S4.
The structure of NiCo2S4@NCNT was characterized by X-ray diffraction (XRD) and the results are shown in Figure 2a. The typical diffraction peaks appeared at 2θ of 26.5°, 31.7°, 38.1°, 50.3°, and 55.1° are assigned to the (220), (331), (400), (511), and (440) crystal planes of NiCo2S4 (JCPDS No. 20-0782), respectively.[38] It is noted that 9
the peak intensities of the NiCo 2S4 are increased with the introduction of CNT and/or NCNT, which may result from the CNT could provide more active sites to support the NiCo2S4 grow. However, the diffraction peaks of CNT are not observed probably because the surface of CNT were coated with NiCo2S4 particles. Therefore, Raman spectrum was performed to confirm the existence of CNT and the results are presented in Figure 2b. Evidently, two characteristic peaks of the D (1350 cm-1) and G band (1590 cm-1) originates from the CNT were identified in the hybrid of NiCo2S4@NCNT and the relative strength (ID/IG) between the D and G bands corresponds to the degree of graphitization. [39] The ID/IG value of NiCo2S4@NCNT is about 0.91 which was higher than that of NiCo 2S4@CNT (about 0.79) and CNT (about 0.45). This phenomenon indicates the defect rich characteristic caused by the N doping in the sp2 carbon structure, which is due to the use of ammonium hydroxide during the hydrothermal process.[40-41] The N2 adsorption-desorption test was performed to evaluate the specific surface area of the NiCo2S4@NCNT. As shown in the Figure 2c, the specific surface areas of the NiCo2S4@NCNT, NiCo2S4@CNT and NiCo2S4 were calculated to be 75.34, 73.47 and 35.12 m2 g-1, respectively. The pore-size distribution structures were further revealed by applying the BJH method to the adsorption branch of the isotherm (Figure 2d). The peaks located at 2.35 and 45.83 nm reveal the co-exist of meso-pore and marc-pore. Such textural characteristics could provide rich electro-active sites for the energy storage and short diffusion paths for ion and electron transports.
10
Figure 3. The full-scan (a) and the high-resolution Ni 2p (b), Co 2p (c), S 2p (d), N 1s (e) and C 1s (f) XPS spectra of NiCo2S4@NCNT.
The surface species and chemical states of NiCo2S4@NCNT were explored further by XPS measurements. The survey spectrum of the NiCo2S4@NCNT confirms the presence of C 1s, N 1s, O 1s, Ni 2p, Co 2p and S 2p peaks (Figure 3a), consisting with the results of XRD and Raman. The high-resolution XPS spectrum of Ni 2p can be mainly deconvoluted into two spin-orbit doublets with shakeup satellites (marked as “Sat.”), ascribing to Ni 2p3/2 (855.6 eV) and Ni 2p1/2 (873.1 eV), respectively (Figure 3b), suggesting the coexistence of Ni2+ and Ni3+.[42] Meanwhile, there is predominant Co 2p3/2 and Co 2p1/2 peaks centered at 780.8 and 795.9 eV with a spin-orbit splitting of 15.1 eV, suggesting the coexistence of Co 2+/Co3+ species (Figure 3c).[43-44] Moreover, S 2p characteristic photoelectron peaks were found to appear at 163.8 (S 2p1/2) and 162.0 eV (S 2p3/2), respectively (Figure 3d). The peak of S 2p1/2 is typical of a metal-sulphur bond, and the peak of S 2p3/2 is attributed to the sulfur ions (S2-) in a low coordination which on the surface of the hybrid. [45] These results suggest that the 11
Ni, Co, S match well with the reported data of NiCo 2S4 and the NiCo2S4 nanoparticles is assembled onto the NCNT. In addition, the pyridinic N (398.6 eV), pyrrolic N (399.8 eV), graphitic N (400.9 eV) and C-N (285.7 eV) are clearly shown at the high-resolution N and C 1s XPS spectra of NiCo2S4@NCNT (Figure 3e and 3f),[46-48] which suggests that the N are successfully introduced into the CNT and more active sites are provided on the surface of NiCo2S4@NCNT.
Figure 4. The CV (a) and GCD (b) curves of NiCo2S4, NiCo2S4@CNT and NiCo2S4@NCNT at 20 mV s-1 and a current density of 1 A g-1, respectively; The CV (c) and GCD (d) curves of NiCo2S4@NCNT at different scan rates and different current densities; The Specific capacity (e) and Nyquist plots (f) of NiCo2S4, NiCo2S4@CNT and NiCo2S4@NCNT NiCo2S4; The capacitance retention (g) of NiCo2S4@NCNT at a current density of 20 A g-1 after 5000 cycle numbers; (h) The part of charge and discharge curves during the cycling progress.
To explore the superiority of electrochemical performances of the samples, the CV and GCD measurements were first performed in three-system electrode equipped with 6 M KOH solution as electrolyte, the compared CV curves of NiCo2S4@NCNT, NiCo2S4@CNT and NiCo2S4 recorded at a scan rate of 20 mV s-1 from 0 to 0.5 V are shown in Figure 4a. Obviously, the NiCo2S4@NCNT exhibited the most promising 12
capacitance performance in terms of its largest integrated area, indicating the robust faradaic behavior as battery-type electrode materials. Besides, the CV curves show a pair
redox peak,
corresponding
to
the transformation of
the
following
equations:[20,23,49] NiS + OH- NiSOH +e
( 10 )
CoS+OH- CoSOH +e
(11)
CoSOH+OH- CoSO+H2O+e
(12)
The GCD curves measured at a current density of 1 A g -1 were conducted to further evaluate the battery-type properties of the as-synthesized samples. It can be clearly observed that the NiCo 2S4-CNT@N electrode displays the longest charge-discharge time, demonstrated the superior battery-type performance, further proved that doping nitrogen (N) species into carbon materials is another strategy of improving energy storage ability of electrode materials. Meanwhile, the GCD curves displayed a pair distinct plateau region, precisely agree with the results of CV curves, which further verified the battery-type feature. The CVs of NiCo2S4@NCNT record at various scan rates are performed in Figure 4c. With the scan rates increased from 5 mV s-1 to 100 mV s-1, the anode peak moves towards high potential and the cathode peak moves towards low potential at the same time, which is attributed to the polarization effect of electrode. In addition, the peak current densities linearly increase with the scan rates and the redox shapes remind at high scan rates, indicating the rapid redox reaction of NiCo2S4@NCNT.[50] The GCD measurements were also performed at different current densities to further evaluate the electrochemical performance of the 13
as-prepared N-doped electrodes, and the results are shown in Figure 4d. The specific capacity was calculated according to the Eq. (1) and the results are summarized in Figure 4e. Apparently, the specific capacity of NiCo2S4@NCNT, NiCo2S4@CNT and NiCo2S4 are 783.5, 667, and 519 C g-1 at 1 A g-1, respectively, and decrease to be 584.5, 404.5, and 250.5 C g-1 at relativity high current density of 50 A g-1. In this respect, the capacity of NiCo2S4@NCNT electrode is much higher than the remaining electrodes and as well as the literature data summarized in Table S1 in the support information due to N dopant could enhance the electrical conductivity of CNT that act as conducting tunnel for charge transfer between CNT and NiCo2S4 and that is enhance the synergistic effect between them. The Figure 4f shows the EIS measurement performed at a frequency range from 0.01 Hz to 100 K Hz, and the inset is an expanded view at the high-frequency region. NiCo2S4@NCNT electrode show a very low equivalent series resistance, the value of the Rct about (0.16 Ω) as compared to NiCo2S4@CNT (0.21 Ω), and NiCo2S4 (0.32 Ω) (Figure S4), indicating that the NiCo2S4@NCNT sample have a small resistance and fast ion response at high-frequency region which is attributed to N dopant. Meanwhile, the NiCo2S4@NCNT sample has a more ideal straight line at high-frequency region, indicating a lower diffusion resistance.[51] Figure 4g show the long cyclic life test of the NiCo2S4@NCNT which was monitored by using galvanostatic charge-discharge measurement at a current density of 20 A g-1. After 5000 cycles, the hybrid NiCo2S4@NCNT electrode retains 86.46 % of the initial capacitance (Figure 4g), thus showing the outstanding structural stability. The part of charge and discharge progress 14
was shown in Figure 4h. The present work provides relativity higher specific capacitance and better rate capability even compared with other reported literatures (Table S1). In addition, the GCD and CV of NiCo 2S4@CNT and NiCo2S4 were tested with different current densities and scan rates which shown in the Figure S5a-5d, the specific capacity of NiCo 2S4@CNT and NiCo2S4 are 667 and 519 C g-1 at 1 A g-1, respectively, and decrease to be 404.5, and 250.5 C g-1 with current density of 50 A g-1. Moreover, NiCo2S4@CNT and NiCo2S4 was characterized by using galvanostatic charge-discharge measurement up to 5000 cycles at a scan rate 10 A g-1 as shown in Figure S5e. The capacitance decreased with the increasing of cycle number and maintained about 76.6 and 53.4% from the original after 5000 cycles.
Figure. 5 The CV (a) and GCD (b) curves of NiCo2S4@NCNT//AC asymmetric supercapacitor collected in different scan rate and different current density. (c) Energy density and power density of the asymmetric supercapacitor NiCo2S4@NCNT//AC; (d) The capacitance retention of NiCo2S4@NCNT//AC at a current density of 10 A g-1. 15
For further exploration the practical application, we used the NiCo2S4@NCNT electrode as positive electrode and AC as the negative electrode to fabricate an HSC. Activated carbon is an ideal supercapacitor electrode due to good conductivity and excellent electrochemical stability. The mass loading of AC and NiCo2S4@NCNT was according to the principle of charge balance. It is well known that the AC electrode behaved as an ideal EDLC with a potential window from -1.0 to 0 V, combined with the potential of NiCo2S4@NCNT from 0 to 0.5 V, the working voltage window increased to 1.5 V. Figure 5a show the CV curves of the HSC recorded at various scan rates. The shape of CV curves is nearly rectangular with a redox peak, demonstrating that the device exhibited nearly ideal battery-like electrochemical capacitive behavior. The superior electrochemical performance of the device was further confirmed by galvanostatic charge-discharge measurement and the values of specific capacity are calculated to be 226.7, 218, 199.5, and 189 C g-1 at the current densities 1, 2, 5, and 10 A g-1, respectively. When the current density come to 20 A g-1, a high specific capacity value of 177.8 C g-1 still could be obtained. Moreover, the rate capability with each current density was shown in the Figure 5b, which also indicates the remarkable capability of the device for high-rate energy storage applications. The energy
density
and
power
density
of
asymmetric
supercapacitor
NiCo2S4@NCNT//AC were calculated from galvanostatic discharge curves of Figure 5c. Notably, the device achieved a maximum energy density of 49.75 Wh kg -1 at a power density of 774.65 W kg-1 which is much higher than those of the latest reported NiCo2S4-based asymmetric and/or symmetric supercapacitors.[25,52-57] This high 16
energy density can be attributing to the active nanoparticles of NiCo 2S4 well assembled on the surface of N doped carbon nanotube and the good match of NiCo 2S4 with
the
N-doped
NiCo2S4@NCNT//AC
CNT. device
Furthermore, was
the
cycling
characterized
by
performance using
of
the
galvanostatic
charge-discharge measurement up to 3000 cycles at a scan rate 10 A g-1 as shown in Figure 5d. The capacitance decreased with the increasing of cycle number and maintained about 89.8 % from the original after 3000 cycles. Conclusions In this paper, a novel electrostatic self-assembly combined one-step hydrothermal method is performed to prepare NiCo2S4@NCNT hybrid. Benefit from its unique structure, that is the high special surface area of CNTs will shorten the diffusion path of ion and N dopant could validly reduce the electron transportation resistance, when employ NiCo2S4@NCNT hybrid as a battery-type electrode to assemble HSC, it demonstrated an outstanding electrochemical performance with a high energy density of 49.75 Wh kg-1 at a power density of 774.65 W kg-1. The device also possessed amazing cycle stability, therefore, the as-prepared HSC device is a promising candidate for high-performance energy storage devices in reality. ACKNOWLEDGMENT We gratefully acknowledge the financial support of this research by National Nature Science Foundation of China (21503055), the Hong Kong Scholars Programs (Grant No. XJ2016046), the China Postdoctoral Science Foundation (2015M571390), the Natural Science Foundation of Heilongjiang Province of China (QC2015015) and the 17
Heilongjiang Postdoctoral Fund (LBHZ14054, LBH-TZ0609).
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25
Highlight
NiCo2O4 nanoparticles are uniformly distributed on the surface of N-doped CNTs
The NiCo2S4@NCNT electrode exhibits high specific capacitances and cycle stability.
NiCo2O4@NCNT//AC hybrid supercapacitor exhibits high energy desity of 49.75 Wh kg-1.
26