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MOF–derived hollow double–shelled NiO nanospheres for high–performance supercapacitors
Accepted Manuscript MOF–derived hollow double–shelled NiO nanospheres for high–performance supercapacitors Meng–Ke Wu, Chen Chen, Jiao–Jiao Zhou, Fei–Yan Yi, Kai Tao, Lei Han PII:
S0925-8388(17)33607-1
DOI:
10.1016/j.jallcom.2017.10.171
Reference:
JALCOM 43568
To appear in:
Journal of Alloys and Compounds
Received Date: 15 September 2017 Revised Date:
20 October 2017
Accepted Date: 21 October 2017
Please cite this article as: Meng–Ke. Wu, C. Chen, Jiao–Jiao. Zhou, Fei–Yan. Yi, K. Tao, L. Han, MOF– derived hollow double–shelled NiO nanospheres for high–performance supercapacitors, Journal of Alloys and Compounds (2017), doi: 10.1016/j.jallcom.2017.10.171. 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.
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Graphical Abstract
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MOF–derived hollow double–shelled NiO nanospheres for high–performance supercapacitors
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Meng–Ke Wu, Chen Chen, Jiao–Jiao Zhou, Fei–Yan Yi, Kai Tao, Lei Han*
State Key Laboratory Base of Novel Functional Materials and Preparation Science, School of
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Materials Science & Chemical Engineering, Ningbo University, Ningbo, Zhejiang 315211, China
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* Corresponding author. E–mail addresses: [email protected]
Abstract
Hollow double–shelled NiO nanospheres have been successfully prepared by the calcination of Ni–based MOF precursor at proper temperatures. The morphology and phase structure of NiO
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samples were characterized by SEM, TEM, XRD and FTIR. Electrochemical studies indicate that NiO nanospheres obtained under different conditions have distinct electrochemical performances. Hollow double–shelled NiO nanospheres calcined at 400℃ (N400) exhibit the best charge
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storage with a specific capacitance of 473 F g–1 at the current density of 0.5 A g–1 and with 94% capacitance retention even after 3000 cycling tests. For this merits, the N400//active carbon (AC)
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asymmetric supercapacitor (ASC) was assembled to examine the practical application of the N400 sample, which presents high energy density of 21.4 Wh kg–1 at power density of 375.8 W kg–1. Particularly, the N400//AC ASC exhibits outstanding cycling stability, and its capacitance retention can still reach up to 92.3% after 3000 cycles. These results indicate that the N400 sample is a promising electroactive candidate for supercapacitors.
Keywords:
Metal–organic
Supercapacitors
frameworks;
NiO;
Hollow
double–shelled
nanospheres;
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1. Introduction Over the past decades, with the increasing depletion of fossil fuels, the problems of environmental pollution and energy shortage have called for intense study on energy storage and
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transformation of alternative energy sources [1,2]. Supercapacitors, also known as electrochemical capacitors, have received much attention in recent years due to its high power density and excellent cycle stability [3–5]. The supercapacitors generally include pseudocapacitors and electric double layer capacitors (EDLCs). Compared to EDLCs forming a double layer on the electrode
higher energy density and specific capacitance.
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surface [6,7], pseudocapacitors based on the reversible redox reactions usually present much
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In particular, pseudocapacitors based on various types of ruthenium oxides have been widely studied due to its high specific capacitance and good electrochemical stability [8]. However, the costless and toxic of ruthenium limit their commercial applications. Thus, many efforts have been made to explore the desired electrode materials with good capacitive performance, such as NiO [9], Co3O4 [10], MnO2 [11, 12], Co(OH)2 [13], Ni(OH)2 [14], etc. Among these candidates, NiO is
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more competitive owing to its low cost, large surface area, and thermal stability. In addition, the theoretical specific capacitance of NiO can reach up to 2584 F g–1 within 0.5 V [15], indicating its excellent capacitive character. In recent years, NiO has been received extensive research interests,
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especially the NiO with multi–shelled hollow spheres have been regarded as a promising material for supercapacitors [16,17].
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To date, various methods have been used to synthesized nickel oxides, such as chemical precipitation [18], electrodeposition [19], sol–gel technique [20] and templating method [21]. Among these synthetic methods, the templating method is regarded as a universal and effective method to fabricate the metal oxides with highly uniform size and morphology [22–25]. Recently, metal–organic frameworks (MOFs) as porous crystalline materials [26] have been widely used as templates or precursors for the preparation of metal oxides [27–32]. For instance, porous Co3O4 microflowers were obtained by calcinating MOF microcrystals [10], porous spindle–like α–Fe2O3 were prepared by using the MIL–88–Fe as a template [22], porous CuO particles were synthesized through thermal decomposition of HKUST–1 [33]. In comparison with the metal oxides obtained by other approaches, the MOF–derived metal oxides show highly uniform
ACCEPTED MANUSCRIPT porosity, which is of significance in achieving the improved electrochemical performance. However, there is a great challenge to obtain well–defined MOF precursors and maintain the original morphology after calcination. In this work, we successfully synthesized a Ni–based MOF nanosphere by a facile
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solvothermal method. Then by controlling the calcination conditions, a highly uniform NiO hollow nanosphere with a core–in–double–shell structure was obtained. The electrochemical performances of NiO nanospheres calcined at 400, 500, and 600 ℃ have been investigated as supercapacitor electrodes, respectively. The NiO calcined at 400 ℃ (N400) exhibits good
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electrochemical performance with specific capacitance of 473 F g–1 at a current density of 0.5 A g–1 and excellent cycling stability with 94% capacitance retention after 3000 cycles tests.
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Furthermore, an asymmetric supercapacitor (ACS) based on N400 nanospheres shows the energy density of 21.4 Wh kg–1 at power density of 375.8 W kg–1. 2. Experimental Section 2.1 Materials preparation
All chemicals of reagent grade in the experiment were commercially available and used
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without further purification. In the typical procedure, 20 ml of 0.02 M Ni(NO3)2·6H2O in N,N–dimethylformamide (DMF) solution, 25 mg of 1,4–benzenedicarboxylic acid and 6 ml of ethylene glycol were mixed at room temperature with vigorous stirring for 30 min. Then the
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solution was transferred into a 50 ml Teflon–lined autoclave maintained at 120 ℃ for 6 h. After cooling to room temperature gradually, the light green precipitation was obtained by
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centrifugation and collected after washing with DMF and anhydrous ethanol several times, and then dried at 60 ℃ for 12 h. Subsequently, the above virescent precipitation was calcinated at 400 ℃, 500 ℃, 600 ℃ for 1 h in air with a ramp rate of 1 ℃ min–1, respectively. Then it was found that the color of the product changed from virescent to black. After cooling to room temperature, the black powders were obtained. 2.2 Characterization The phase structure of the as–prepared samples were collected by a Bruker D8 advance X–ray powder diffractometer using Cu Ka radiation (λ = 0.15418 nm). FT–IR spectra was measured using a FT/IR–4600 (Jasco, Japan). The morphology of the samples were recorded by
ACCEPTED MANUSCRIPT field–emission scanning electron microscopy (FE–SEM, Hitachi, S–4800) and transmission electron microscopy (TEM, JEOL, 2010F). The thermal stability of the precursor was performed by
employing
thermogravimetric
(TG)
on
SII
TG/DTA7300
thermal
analyzer.
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adsorption/desorption was examined by Brunauere–Emmette–Teller (BET) measurements using
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ASAP 2020 analyzer. The electrochemical tests were performed by an electrochemical analyzer system, CHI660E (Chenhua, Shanghai, China) using a three–electrode cell in 3.0 M KOH aqueous solution. 2.3 Electrochemical measurements
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The electrochemical measurements of the NiO electrodes were studied based on cyclic voltammetry (CV), galvanostatic charge/discharge (GCD) curves and electrochemical impedance
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spectroscopy (EIS) using a three–electrode cell, containing a Hg/HgO as the reference electrode, a Pt foil as the counter electrode and active materials coated on nickel foam as the working electrode. The working electrode was fabricated as follows: A mixture containing 80 wt.% of NiO nanospheres, 10 wt.% acetylene black and 10 wt.% poly(vinylidinedifluoride) (PVDF) with a little isopropanol to make a homogeneous slurry. Then the slurry was coated on the nickel foam
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substrates (1 cm2) and dried at 80 ℃ for 12 h.
The electrochemical behavior of the asymmetric supercapacitor was then performed in a two–electrode cell, where the NiO positive electrode and commercial AC negative electrode were
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pressed together and separated by a piece of cellulose paper. The electrolytes of the above electrochemical tests were 3.0 M KOH aqueous solution on an electrochemical analyzer system,
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CHI660E (Chenhua, Shanghai, China).
3. Results and discussion
The Ni–MOF precursor was synthesized by a facile solvothermal method at 120 ℃ for 6 h
in the oven, resulting in green Ni–MOF nanospheres. The power X–ray diffraction (XRD) pattern of the MOF precursor was investigated, as shown in Fig. S1a, the diffraction peaks are well in agreement with the reported Ni–based MOF materials. Meanwhile, the FT–IR spectrum (Fig. S1b) of the precursor further confirmed that the product was successfully synthesized by comparing with the Ni–MOF material in accordance with the reported literature [34]. The morphology of Ni–MOF precursor was observed by SEM and shown in Fig. 1a, which reveals the uniform and
ACCEPTED MANUSCRIPT monodisperse spheres with an average particle size about 500 nm. The TEM image in Fig. 1b confirms that the Ni–MOF precursor is composed of the solid spheres without pores. TGA curve of the MOF precursor (Fig. S2) exhibits that it decomposes with the increasing temperature until 400 ℃ in air. Therefore, the Ni–MOF precursor was calcined in air at 400, 500
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and 600 ℃ for 1 h to obtain the NiO nanospheres, which are named as N400, N500, and N600, respectively. As expected, the spherical morphology of the precursor was well maintained after calcination in air (Fig. 1c and 1d). The size of obtained NiO is smaller than that of MOF precursor, with the average diameter is about 300 nm, indicating that the product has shrunk during the heat
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treatment. Upon further rise in temperature, the surface of samples become more and more coarse and closely packed nanoparticles are formed on the surface, as shown in Fig. S3. The interior
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structure of NiO nanospheres were further studied by TEM. As shown in Fig. 1e and 1f, it can be observed that N400 nanospheres comprise a core and a double–shell, the average diameters of the outer and inner shells as well as the solid core are about 300, 150 and 75 nm, respectively. Although the inner shell is close to the solid core, a clear gap between them is still observed, indicating the presence of core–in–double–shell hollow structure. While with the increase of
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calcining temperature, the solid core is gradually fade away and hollow nanospheres are formed, as shown in Fig. S4. And Fig. S4c and 4d show that the hollow nanospheres have a completely void interior. This unique structure may be ascribed to the heating rate during the calcination
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process. And it has been reported that the slower the heating rate, the more conducive to the formation of the complex interior structures [35].
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The XRD patterns of as–synthesized NiO products are exhibited in Fig. 2a, all peaks are corresponding to the diffractions (110), (200), (200), (311) and (222) of NiO structures (JCPDS Card No.47–1049) The peaks become more intense and sharp with the increase of temperature, indicating that the crystallinity is getting better and better. In addition, the XRD patterns show that no impurity diffraction peaks are found, suggesting the complete conversion of MOF precursors into NiO at 400 ℃ in air. The electrochemical reaction activity of NiO depends on its crystallinity, which is related to the calcination temperature. And many works have confirmed the good capacitance of NiO calcined at low temperature [36–38]. To further evaluate the specific surface areas and pore size distribution of samples, Brunauer–Emmett–Teller (BET) adsorption/desorption measurements were performed and shown
ACCEPTED MANUSCRIPT in Fig. 2b–d. The nitrogen adsorption isotherm of N400 exhibits an obvious hysteresis loop in the range of ca. 0.5–1.0 P/P0, indicating the existence of abundant mesopores in the sample. The BET surface areas were determined to be 66.8, 8.8, and 5.0 m2 g–1 for the N400, N500, and N600, respectively. The higher surface area of the N400 can enable an efficient contact of electrode
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materials with electrolyte. The corresponding Barrett–Joyner–Halenda (BIH) pore size distribution curves (inset in Fig. 2b–d) demonstrate that N400 possesses uniform mesopores with an average pore size of 2.4 nm, while those of N500 and N600 are not uniform, and exhibiting extensive pore sizes of 2–4 nm and 5–10 nm. It is the narrow pore size of N400 that leads to a high surface area.
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Such porosity of hollow NiO nanospheres not only can alleviate the volume change during the charge–discharge process, but also can facilitate the transfer of electrons and ions at the
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electrode/electrolyte interface [39].
To identify the capacitive characteristics of hollow NiO nanospheres, the related electrochemical tests of N400–N600 electrodes were measured in 3 M KOH aqueous solution as an electrolyte (Fig. 3 and Fig. S5). Fig. 3a exhibits the CV curves of N400, N500, and N600 at 20 mV s–1. Apparently, the current density of N400 is much larger than that of N500 and N600.
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Because the specific capacitance is directly proportional to the area of the CV curves, N400 electrode demonstrates much higher charge storage capability compared to N500 and N600. A pair of strong redox peaks can be obviously seen in the CV curves, revealing that the capacitance of
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NiO mainly results from pseudocapacitance. The couple redox peaks correspond to a reversible redox reaction on the surface of the electrode and have been described according to the following
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eqn (1): NiO + OH– ↔ NiOOH + e–. Fig. 3b exhibits CV curves at a scan rate of 5–50 mV s–1 for N400 electrode. With the increase of the scan rates, the cathodic and anodic current densities improve accordingly, while no distinct change in the shape of CV curves expect for the polarization at high scan rate, showing the good rate capability of the N400 electrode. Fig. 3c exhibits the GCD curves of N400, N500, and N600 in the potential range of 0–0.45 V at 0.5 A g–1. Obviously, the charge–discharge time of N400 is much longer than N500 and N600, suggesting the specific capacitance is the largest in the N400 case. Fig. 3d presents the GCD curves of N400 electrode at various discharging current densities. The defined voltage plateaus in the charge–discharge curves imply the properties of pseudocapacitance, which corresponds to the CV results in Fig. 3b.
ACCEPTED MANUSCRIPT The specific capacitance (C, F g–1) can be calculated from the discharge curves based on the following eqn (2): C = (I ∆t) / (m ∆V), where I (A) is the discharge current, and ∆t (s), m (g), ∆V (V) represent the total discharge time, the mass of active materials and the potential range of discharge, respectively. Here the mass of active material loading on electrode was 3 mg cm–2. Fig.
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3e shows the corresponding specific capacitance of N400–N600 at current densities of 0.5, 1, 2, 3, 5, 10 A g–1. The specific capacitance of N400 electrode at a current density of 0.5 A g–1 is as high as 473 F g–1, and even remains 332.8 F g–1 while the current density increasing to 10 A g–1. The capacitance retention from 0.5 to 10 A g–1 for N400 is 70%. By contrast, the N500 and N600
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exhibit inferior capacitance properties (309 and 144 F g–1 at 0.5 A g–1). At the current density of 10 A g–1, the specific capacitance of N500 and N600 can also be maintained at 188.8 and 111.2 F g–1,
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and the corresponding retention rates are 61% and 77%, respectively. The excellent capacitance and rate capability of N400 can be ascribed to its unique structure. As redox reaction occurred at the interface of NiO nanoparticles and electrolyte, high specific capacitance could be achieved in the condition of high specific surface area of N400 with abundant channels for diffusion of OH– ions. In addition, the hollow core–in–double–shell structure of N400 can alleviate the volume
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change during the charge–discharge process and accelerate the transfer of electrons so as to facilitate the reversible faradaic reactions. Thus, specific capacitance of N400 is the highest compared to the N500 and N600. This experimental phenomenon is consistent with the previous reports [5,38]. Furthermore, our results demonstrate that N400 possesses more excellent
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capacitance and rate capability compared with other cases, such as NiO nanotubes (405 F g–1), nanoporous NiO film (167.3 F g–1), mesoporous NiO (165 F g–1), and NiO nanoplatelets (108 F
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g–1) [40–43], as shown in Table 1.
To further evaluate the exact electrical conductivity of electrodes, the electrochemical
impedance spectroscopy (EIS) has been measured in a frequency range from 0.01 Hz to 100 KHz and the corresponding Nyquist plots are shown in Fig. 3f. It is obviously that all the samples exhibit a straight line in the low–frequency region, meaning a good capacitive behavior. And the bulk solution resistance (Rs), derived from the intercept at the real axis, are 5.7, 6.3, and 8.0 Ω for N400, N500 and N600, respectively. In the high–frequency range, the semicircle diameter represents the charge transport resistance (Rct) which are determined to be 0.52, 1.48, 1.12 Ω, respectively, indicating that N400 electrode has the lowest charge transport resistance (Rct). As we
ACCEPTED MANUSCRIPT all know that the efficiency of ion diffusion and charge transport are limiting factors for the specific capacity of the supercapacitor. It can be concluded that the low charge transfer resistance results in the high specific capacity of N400 electrode. In addition, the cyclic stability is a significant criterion for supercapacitors. The cycling
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performance of N400, N500 and N600 electrodes were measured by continuous charge–discharge cycling at a current density of 5 A g–1 for 3000 cycles. As shown in Fig. 4, the corresponding capacitance retention of N400, N500, N600 are about 94%, 85% and 78%, respectively. The slight reduce of the specific capacitance of N400 may be due to a little dropping of active materials from
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the electrode. However, it is still better than other NiO materials reported previously [40–43]. Based on the above results, N400 is a desired electrode material for supercapacitor applications.
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To further investigate the practical application of NiO nanospheres, an asymmetric supercapacitor (ACS) based on N400 and active carbon (AC) materials was fabricated using CV and galvanostatic charge–discharge (GCD) measurements in 3 M KOH solution (Fig. 5). In addition, the electrochemical performance of AC electrode was also investigated using a three electrode system in 3 M KOH electrolyte (Fig. S6a–c). The specific capacitance of AC electrode is
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180 F g–1 at a current density of 1 A g–1, which was calculated from the discharge curves. It is significant to balance the charges stored at positive electrode (Q+) and negative electrode (Q–) for a supercapacitor with high operating voltage and high energy density [44]. The charges (Q) stored
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at each electrode can be calculated from the following eqn (3): Q = C × ∆V × m. To realize Q+ = Q–, the mass ratio between the N400 and AC electrodes can be determined by the eqn (4): m+ /
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m–= (C– × ∆V–) / (C+ × ∆V+). In this case, the optical mass ratio (m(N(400)/m(AC)) is about 0.8 for the ASC. The operating potential of the ASC was evaluated by using CV tests at the scan rate of 20 mV s–1 in different voltage windows, as shown in Fig. S6d. When the potential window is extended up to 1.5 V, the significant redox humps can be observed, indicating the pseudocapacitive properties of N400//AC ASC deriving from N400 electrode. After 1.5 V, there is a little distortion in the curve. Hence, the operating voltage window can be determined at 0–1.5 V. The CV curves of the N400//AC ASC at different scan rates from 5 to 50 mV s–1 are shown in Fig. 5a. With the increases of scan rate, the current density slowly increased with no significant distortion, indicating the great charge–discharge performance of the ASC device. Fig. 5b shows the GCD curves at different current densities ranging from 0.5 to 10 A g–1. The specific
ACCEPTED MANUSCRIPT capacitance of the ASC was calculated based on the total mass of active materials of each electrode (Fig. 5c). The highest specific capacitance is up to 68.5 F g–1 at 0.5 A g–1, which only declined to 40 F g–1 at a high current density of 10 A g–1. Moreover, no clear IR drop was found in the discharge curves, indicating the low internal resistance of the supercapacitor [45]. The cycling
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stability of the ASC was further evaluated at a current density of 5 A g–1 (Fig. 5d). Apparently, the ASC demonstrates admirable electrochemical cycling performance after 3000 cycles, and the capacitance retention can be maintained at 92.3%, indicating that there is a highly reversible redox reaction between the active materials and electrolyte.
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The power density and energy density are two key parameters to evaluate the energy storage performance of supercapacitors. The energy density (E) and power density (P) can be calculated
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by eqn (5) and eqn (6):
E=
1 / 2C∆V 2 (5) 3 .6
P=
3600 E ∆t
(6)
where E (Wh kg–1), P (W kg–1), C (F g–1), ∆V (V), ∆t (s) are the energy density, the power density,
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the specific capacitance, the range of the potential, and the total discharge time, respectively. To investigate the performance of the N400//AC ASC, a Ragone plot (the energy density vs power density) was carried as shown in Fig. 6. Obviously, the N400//AC ASC exhibits high energy
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density of 21.4 Wh kg–1 at a power density of 375.8 W kg–1, and still remains 12.5 Wh kg–1 even at a high power density of 7500 W kg–1, which outperforms many previously reported ASCs, such
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as NiO//Carbon (11.6 Wh kg–1 at 28 W kg–1) [46], NiCo2O4–MnO2//AG (9.4 Wh kg–1 at 2500 W kg–1) [47], Graphene–NiCo2O4//AC (7.6 Wh kg–1 at 5600 W kg–1) [48], Ni–Co oxide//AC (7.4 Wh kg–1 at 1903 W kg–1) [49], and MnO2 nanowire/Graphene//Graphene (7.0 Wh kg–1 at 5000 W kg–1) [50]. All the experimental results vividly indicate that the N400 hollow double–shelled nanospheres possess a potential application for high–performance supercapacitor.
4. Conclusions In conclusion, hollow core–in–double–shell NiO nanospheres have been successfully fabricated by the calcination of the Ni–based MOF precursors. As–synthesized NiO nanospheres
ACCEPTED MANUSCRIPT calcined at different temperatures have distinct surface areas and electrical conductivities, which play vital roles to the redox reaction occurred at the surface of the active materials. By contrast, N400 shows a specific capacitance of 473 F g–1 at the current density of 0.5 A g–1 and manifests a good retention after 3000 cycling tests. Meanwhile, the N400//AC ASC was assembled to further
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confirmed the potential application of the N400, which presents high energy storage capacity and exhibits excellent cycling performance. These results indicate that N400 is a promising electroactive candidate for supercapacitors. Furthermore, the MOF–derived metal oxides possess highly uniform porosity, which plays a crucial role in achieving the increases of electrochemical
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performance. Hence, the development of MOF–derived metal oxides possessing high surface area
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and well–defined shape is a promising strategy for surpercapacitor materials.
Acknowledgements
This work was supported by the National Natural Science Foundation of China (No. 21471086; No. 51572272), the Science and Technology Department of Zhejiang Province (No. 2017C33007), the Natural Science Foundation of Ningbo (No. 2017A610062; No. 2017A610065), and the K.C.
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Wong Magna Fund in Ningbo University.
Appendix A. Supplementary data
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Supplementary data associated with this article can be found, in the online version, at
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http://dx.doi.org/xxx.
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ACCEPTED MANUSCRIPT mesoporous NiO nanorod for high–performance lithium ion batteries, Electrochim. Acta 213 (2016) 351–357. [28] W. X. Guo, W. W. Sun, Y. Wang, Multilayer CuO@NiO hollow spheres: microwave–assisted metal–organic framework derivation and highly reversible structure–matched stepwise lithium
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[35] L. Shen, L. Yu, X.–Y. Yu, X. Zhang, X. W. Lou, Self–templated formation of uniform NiCo2O4 hollow spheres with complex interior structures for lithium–ion batteries and supercapacitors, Angew. Chem. Int. Ed. 54 (2015) 1868 –1872. [36] W. Xing, F. Li, Z. F. Yan, G. Q. Lu, Synthesis and electrochemical properties of mesoporous nickel oxide, J. Power Sources 134 (2004) 324–330. [37] M. Q. Wu, J. H. Gao, S. R. Zhang, A. Chen, Synthesis and characterization of aerogel–like mesoporous nickel oxide for electrochemical supercapacitors, J. Porous Mater. 13 (2006)
ACCEPTED MANUSCRIPT 407–412. [38] J. W. Lee, T. Ahn, J. H. Kim, J. M. Ko, J.–D. Kim, Nanosheets based mesoporous NiO microspherical structures via facile and template–free method for high performance supercapacitors, Electrochim. Acta 56 (2011) 4849–4857.
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[41] M. S. Wu, H. H. Hsieh, Nickel oxide/hydroxide nanoplatelets synthesized by chemical precipitation for electrochemical capacitors, Electrochim. Acta 53 (2008) 3427–3435. [42] S. L. Xiong, C. Z. Yuan, X. G. Zhang, Y. T. Qian, Mesoporous NiO with various hierarchical nanostructures by quasi–nanotubes/nanowires/nanorods self–assembly: controllable preparation and application in supercapacitors, CrystEngComm 13 (2011) 626–632.
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[43] M. S. Wu, Y. A. Huang, C. H. Yang, J. J. Jow, Electrodeposition of nanoporous nickel oxide film for electrochemical capacitors, Int. J. Hydrogen Energy 32 (2007) 4153–4159. [44] J. Yan, Z. Fan, W. Sun, G. Ning, T. Wei, Q. Zhang, R. Zhang, L. Zhi, F. Wei, Advanced
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ACCEPTED MANUSCRIPT level mass loading, Nano Res. 5 (2012) 605–617. [49] C. Tang, Z. Tang, H. Gong, Hierarchically porous Ni–Co oxide for high reversibility asymmetric full–cell supercapacitors, J. Electrochem. Soc. 159 (2012) 651–656. [50] Z. S. Wu, W. Ren, D. W. Wang, F. Li, B. Liu, H. M. Cheng, High–energy MnO2
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nanowire/graphene and graphene asymmetric electrochemical Capacitors, ACS Nano 4 (2010)
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Fig. 1 (a) SEM image of MOF precursor. (b) TEM image of MOF precursor. (c,d) SEM images of N400. (e,f) TEM images of N400 with hollow core–in–double–shell nanospheres.
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Fig. 2 (a) XRD patterns of as–synthesized NiO nanospheres. (b–d) Nitrogen adsorption/desorption isotherms measured at 77 K for N400, N500, and N600, respectively. The insets show the
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corresponding BJH pore size distributions.
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Fig. 3 (a) CV curves of N400, N500, and N600 at a scan rate of 20 mV s–1. (b) CV curves of N400 at different scan rates. (c) GCD curves of N400, N500, and N600 at 0.5 A g–1. (d) GCD curves of N400 at different current densities. (e) Specific capacitance of N400, N500, and N600 at various current densities. (f) EIS curves of N400, N500, and N600 at room temperature in 3.0 M KOH solution.
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Fig. 4 Specific capacitance retention with the number of charging–discharging cycling test at a
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current density of 5 A g–1.
Fig. 5 (a) CV curves of the N400//AC ASC at different scan rates. (b) GCD curves of the
N400//AC ASC at various current densities. (c) The specific capacitances of the N400//AC ASC at different current densities. (d) Cycling performance of N400//AC ASC at a current density of 5 A g–1.
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Fig. 6 Ragone plots of the as–prepared N400//AC ASC compared to the reference.
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1. Hollow double-shelled NiO nanospheres were prepared from MOF precursors. 2. The NiO nanospheres show high specific capacitance.
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3. The NiO nanospheres exhibit excellent cycling performance.
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4. The assembled asymmetric supercapacitor presents high energy density.
S0925-8388(17)33607-1
DOI:
10.1016/j.jallcom.2017.10.171
Reference:
JALCOM 43568
To appear in:
Journal of Alloys and Compounds
Received Date: 15 September 2017 Revised Date:
20 October 2017
Accepted Date: 21 October 2017
Please cite this article as: Meng–Ke. Wu, C. Chen, Jiao–Jiao. Zhou, Fei–Yan. Yi, K. Tao, L. Han, MOF– derived hollow double–shelled NiO nanospheres for high–performance supercapacitors, Journal of Alloys and Compounds (2017), doi: 10.1016/j.jallcom.2017.10.171. 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.
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Graphical Abstract
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MOF–derived hollow double–shelled NiO nanospheres for high–performance supercapacitors
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Meng–Ke Wu, Chen Chen, Jiao–Jiao Zhou, Fei–Yan Yi, Kai Tao, Lei Han*
State Key Laboratory Base of Novel Functional Materials and Preparation Science, School of
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Materials Science & Chemical Engineering, Ningbo University, Ningbo, Zhejiang 315211, China
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* Corresponding author. E–mail addresses: [email protected]
Abstract
Hollow double–shelled NiO nanospheres have been successfully prepared by the calcination of Ni–based MOF precursor at proper temperatures. The morphology and phase structure of NiO
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samples were characterized by SEM, TEM, XRD and FTIR. Electrochemical studies indicate that NiO nanospheres obtained under different conditions have distinct electrochemical performances. Hollow double–shelled NiO nanospheres calcined at 400℃ (N400) exhibit the best charge
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storage with a specific capacitance of 473 F g–1 at the current density of 0.5 A g–1 and with 94% capacitance retention even after 3000 cycling tests. For this merits, the N400//active carbon (AC)
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asymmetric supercapacitor (ASC) was assembled to examine the practical application of the N400 sample, which presents high energy density of 21.4 Wh kg–1 at power density of 375.8 W kg–1. Particularly, the N400//AC ASC exhibits outstanding cycling stability, and its capacitance retention can still reach up to 92.3% after 3000 cycles. These results indicate that the N400 sample is a promising electroactive candidate for supercapacitors.
Keywords:
Metal–organic
Supercapacitors
frameworks;
NiO;
Hollow
double–shelled
nanospheres;
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1. Introduction Over the past decades, with the increasing depletion of fossil fuels, the problems of environmental pollution and energy shortage have called for intense study on energy storage and
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transformation of alternative energy sources [1,2]. Supercapacitors, also known as electrochemical capacitors, have received much attention in recent years due to its high power density and excellent cycle stability [3–5]. The supercapacitors generally include pseudocapacitors and electric double layer capacitors (EDLCs). Compared to EDLCs forming a double layer on the electrode
higher energy density and specific capacitance.
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surface [6,7], pseudocapacitors based on the reversible redox reactions usually present much
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In particular, pseudocapacitors based on various types of ruthenium oxides have been widely studied due to its high specific capacitance and good electrochemical stability [8]. However, the costless and toxic of ruthenium limit their commercial applications. Thus, many efforts have been made to explore the desired electrode materials with good capacitive performance, such as NiO [9], Co3O4 [10], MnO2 [11, 12], Co(OH)2 [13], Ni(OH)2 [14], etc. Among these candidates, NiO is
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more competitive owing to its low cost, large surface area, and thermal stability. In addition, the theoretical specific capacitance of NiO can reach up to 2584 F g–1 within 0.5 V [15], indicating its excellent capacitive character. In recent years, NiO has been received extensive research interests,
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especially the NiO with multi–shelled hollow spheres have been regarded as a promising material for supercapacitors [16,17].
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To date, various methods have been used to synthesized nickel oxides, such as chemical precipitation [18], electrodeposition [19], sol–gel technique [20] and templating method [21]. Among these synthetic methods, the templating method is regarded as a universal and effective method to fabricate the metal oxides with highly uniform size and morphology [22–25]. Recently, metal–organic frameworks (MOFs) as porous crystalline materials [26] have been widely used as templates or precursors for the preparation of metal oxides [27–32]. For instance, porous Co3O4 microflowers were obtained by calcinating MOF microcrystals [10], porous spindle–like α–Fe2O3 were prepared by using the MIL–88–Fe as a template [22], porous CuO particles were synthesized through thermal decomposition of HKUST–1 [33]. In comparison with the metal oxides obtained by other approaches, the MOF–derived metal oxides show highly uniform
ACCEPTED MANUSCRIPT porosity, which is of significance in achieving the improved electrochemical performance. However, there is a great challenge to obtain well–defined MOF precursors and maintain the original morphology after calcination. In this work, we successfully synthesized a Ni–based MOF nanosphere by a facile
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solvothermal method. Then by controlling the calcination conditions, a highly uniform NiO hollow nanosphere with a core–in–double–shell structure was obtained. The electrochemical performances of NiO nanospheres calcined at 400, 500, and 600 ℃ have been investigated as supercapacitor electrodes, respectively. The NiO calcined at 400 ℃ (N400) exhibits good
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electrochemical performance with specific capacitance of 473 F g–1 at a current density of 0.5 A g–1 and excellent cycling stability with 94% capacitance retention after 3000 cycles tests.
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Furthermore, an asymmetric supercapacitor (ACS) based on N400 nanospheres shows the energy density of 21.4 Wh kg–1 at power density of 375.8 W kg–1. 2. Experimental Section 2.1 Materials preparation
All chemicals of reagent grade in the experiment were commercially available and used
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without further purification. In the typical procedure, 20 ml of 0.02 M Ni(NO3)2·6H2O in N,N–dimethylformamide (DMF) solution, 25 mg of 1,4–benzenedicarboxylic acid and 6 ml of ethylene glycol were mixed at room temperature with vigorous stirring for 30 min. Then the
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solution was transferred into a 50 ml Teflon–lined autoclave maintained at 120 ℃ for 6 h. After cooling to room temperature gradually, the light green precipitation was obtained by
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centrifugation and collected after washing with DMF and anhydrous ethanol several times, and then dried at 60 ℃ for 12 h. Subsequently, the above virescent precipitation was calcinated at 400 ℃, 500 ℃, 600 ℃ for 1 h in air with a ramp rate of 1 ℃ min–1, respectively. Then it was found that the color of the product changed from virescent to black. After cooling to room temperature, the black powders were obtained. 2.2 Characterization The phase structure of the as–prepared samples were collected by a Bruker D8 advance X–ray powder diffractometer using Cu Ka radiation (λ = 0.15418 nm). FT–IR spectra was measured using a FT/IR–4600 (Jasco, Japan). The morphology of the samples were recorded by
ACCEPTED MANUSCRIPT field–emission scanning electron microscopy (FE–SEM, Hitachi, S–4800) and transmission electron microscopy (TEM, JEOL, 2010F). The thermal stability of the precursor was performed by
employing
thermogravimetric
(TG)
on
SII
TG/DTA7300
thermal
analyzer.
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adsorption/desorption was examined by Brunauere–Emmette–Teller (BET) measurements using
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ASAP 2020 analyzer. The electrochemical tests were performed by an electrochemical analyzer system, CHI660E (Chenhua, Shanghai, China) using a three–electrode cell in 3.0 M KOH aqueous solution. 2.3 Electrochemical measurements
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The electrochemical measurements of the NiO electrodes were studied based on cyclic voltammetry (CV), galvanostatic charge/discharge (GCD) curves and electrochemical impedance
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spectroscopy (EIS) using a three–electrode cell, containing a Hg/HgO as the reference electrode, a Pt foil as the counter electrode and active materials coated on nickel foam as the working electrode. The working electrode was fabricated as follows: A mixture containing 80 wt.% of NiO nanospheres, 10 wt.% acetylene black and 10 wt.% poly(vinylidinedifluoride) (PVDF) with a little isopropanol to make a homogeneous slurry. Then the slurry was coated on the nickel foam
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substrates (1 cm2) and dried at 80 ℃ for 12 h.
The electrochemical behavior of the asymmetric supercapacitor was then performed in a two–electrode cell, where the NiO positive electrode and commercial AC negative electrode were
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pressed together and separated by a piece of cellulose paper. The electrolytes of the above electrochemical tests were 3.0 M KOH aqueous solution on an electrochemical analyzer system,
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CHI660E (Chenhua, Shanghai, China).
3. Results and discussion
The Ni–MOF precursor was synthesized by a facile solvothermal method at 120 ℃ for 6 h
in the oven, resulting in green Ni–MOF nanospheres. The power X–ray diffraction (XRD) pattern of the MOF precursor was investigated, as shown in Fig. S1a, the diffraction peaks are well in agreement with the reported Ni–based MOF materials. Meanwhile, the FT–IR spectrum (Fig. S1b) of the precursor further confirmed that the product was successfully synthesized by comparing with the Ni–MOF material in accordance with the reported literature [34]. The morphology of Ni–MOF precursor was observed by SEM and shown in Fig. 1a, which reveals the uniform and
ACCEPTED MANUSCRIPT monodisperse spheres with an average particle size about 500 nm. The TEM image in Fig. 1b confirms that the Ni–MOF precursor is composed of the solid spheres without pores. TGA curve of the MOF precursor (Fig. S2) exhibits that it decomposes with the increasing temperature until 400 ℃ in air. Therefore, the Ni–MOF precursor was calcined in air at 400, 500
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and 600 ℃ for 1 h to obtain the NiO nanospheres, which are named as N400, N500, and N600, respectively. As expected, the spherical morphology of the precursor was well maintained after calcination in air (Fig. 1c and 1d). The size of obtained NiO is smaller than that of MOF precursor, with the average diameter is about 300 nm, indicating that the product has shrunk during the heat
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treatment. Upon further rise in temperature, the surface of samples become more and more coarse and closely packed nanoparticles are formed on the surface, as shown in Fig. S3. The interior
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structure of NiO nanospheres were further studied by TEM. As shown in Fig. 1e and 1f, it can be observed that N400 nanospheres comprise a core and a double–shell, the average diameters of the outer and inner shells as well as the solid core are about 300, 150 and 75 nm, respectively. Although the inner shell is close to the solid core, a clear gap between them is still observed, indicating the presence of core–in–double–shell hollow structure. While with the increase of
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calcining temperature, the solid core is gradually fade away and hollow nanospheres are formed, as shown in Fig. S4. And Fig. S4c and 4d show that the hollow nanospheres have a completely void interior. This unique structure may be ascribed to the heating rate during the calcination
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process. And it has been reported that the slower the heating rate, the more conducive to the formation of the complex interior structures [35].
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The XRD patterns of as–synthesized NiO products are exhibited in Fig. 2a, all peaks are corresponding to the diffractions (110), (200), (200), (311) and (222) of NiO structures (JCPDS Card No.47–1049) The peaks become more intense and sharp with the increase of temperature, indicating that the crystallinity is getting better and better. In addition, the XRD patterns show that no impurity diffraction peaks are found, suggesting the complete conversion of MOF precursors into NiO at 400 ℃ in air. The electrochemical reaction activity of NiO depends on its crystallinity, which is related to the calcination temperature. And many works have confirmed the good capacitance of NiO calcined at low temperature [36–38]. To further evaluate the specific surface areas and pore size distribution of samples, Brunauer–Emmett–Teller (BET) adsorption/desorption measurements were performed and shown
ACCEPTED MANUSCRIPT in Fig. 2b–d. The nitrogen adsorption isotherm of N400 exhibits an obvious hysteresis loop in the range of ca. 0.5–1.0 P/P0, indicating the existence of abundant mesopores in the sample. The BET surface areas were determined to be 66.8, 8.8, and 5.0 m2 g–1 for the N400, N500, and N600, respectively. The higher surface area of the N400 can enable an efficient contact of electrode
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materials with electrolyte. The corresponding Barrett–Joyner–Halenda (BIH) pore size distribution curves (inset in Fig. 2b–d) demonstrate that N400 possesses uniform mesopores with an average pore size of 2.4 nm, while those of N500 and N600 are not uniform, and exhibiting extensive pore sizes of 2–4 nm and 5–10 nm. It is the narrow pore size of N400 that leads to a high surface area.
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Such porosity of hollow NiO nanospheres not only can alleviate the volume change during the charge–discharge process, but also can facilitate the transfer of electrons and ions at the
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electrode/electrolyte interface [39].
To identify the capacitive characteristics of hollow NiO nanospheres, the related electrochemical tests of N400–N600 electrodes were measured in 3 M KOH aqueous solution as an electrolyte (Fig. 3 and Fig. S5). Fig. 3a exhibits the CV curves of N400, N500, and N600 at 20 mV s–1. Apparently, the current density of N400 is much larger than that of N500 and N600.
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Because the specific capacitance is directly proportional to the area of the CV curves, N400 electrode demonstrates much higher charge storage capability compared to N500 and N600. A pair of strong redox peaks can be obviously seen in the CV curves, revealing that the capacitance of
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NiO mainly results from pseudocapacitance. The couple redox peaks correspond to a reversible redox reaction on the surface of the electrode and have been described according to the following
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eqn (1): NiO + OH– ↔ NiOOH + e–. Fig. 3b exhibits CV curves at a scan rate of 5–50 mV s–1 for N400 electrode. With the increase of the scan rates, the cathodic and anodic current densities improve accordingly, while no distinct change in the shape of CV curves expect for the polarization at high scan rate, showing the good rate capability of the N400 electrode. Fig. 3c exhibits the GCD curves of N400, N500, and N600 in the potential range of 0–0.45 V at 0.5 A g–1. Obviously, the charge–discharge time of N400 is much longer than N500 and N600, suggesting the specific capacitance is the largest in the N400 case. Fig. 3d presents the GCD curves of N400 electrode at various discharging current densities. The defined voltage plateaus in the charge–discharge curves imply the properties of pseudocapacitance, which corresponds to the CV results in Fig. 3b.
ACCEPTED MANUSCRIPT The specific capacitance (C, F g–1) can be calculated from the discharge curves based on the following eqn (2): C = (I ∆t) / (m ∆V), where I (A) is the discharge current, and ∆t (s), m (g), ∆V (V) represent the total discharge time, the mass of active materials and the potential range of discharge, respectively. Here the mass of active material loading on electrode was 3 mg cm–2. Fig.
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3e shows the corresponding specific capacitance of N400–N600 at current densities of 0.5, 1, 2, 3, 5, 10 A g–1. The specific capacitance of N400 electrode at a current density of 0.5 A g–1 is as high as 473 F g–1, and even remains 332.8 F g–1 while the current density increasing to 10 A g–1. The capacitance retention from 0.5 to 10 A g–1 for N400 is 70%. By contrast, the N500 and N600
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exhibit inferior capacitance properties (309 and 144 F g–1 at 0.5 A g–1). At the current density of 10 A g–1, the specific capacitance of N500 and N600 can also be maintained at 188.8 and 111.2 F g–1,
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and the corresponding retention rates are 61% and 77%, respectively. The excellent capacitance and rate capability of N400 can be ascribed to its unique structure. As redox reaction occurred at the interface of NiO nanoparticles and electrolyte, high specific capacitance could be achieved in the condition of high specific surface area of N400 with abundant channels for diffusion of OH– ions. In addition, the hollow core–in–double–shell structure of N400 can alleviate the volume
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change during the charge–discharge process and accelerate the transfer of electrons so as to facilitate the reversible faradaic reactions. Thus, specific capacitance of N400 is the highest compared to the N500 and N600. This experimental phenomenon is consistent with the previous reports [5,38]. Furthermore, our results demonstrate that N400 possesses more excellent
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capacitance and rate capability compared with other cases, such as NiO nanotubes (405 F g–1), nanoporous NiO film (167.3 F g–1), mesoporous NiO (165 F g–1), and NiO nanoplatelets (108 F
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g–1) [40–43], as shown in Table 1.
To further evaluate the exact electrical conductivity of electrodes, the electrochemical
impedance spectroscopy (EIS) has been measured in a frequency range from 0.01 Hz to 100 KHz and the corresponding Nyquist plots are shown in Fig. 3f. It is obviously that all the samples exhibit a straight line in the low–frequency region, meaning a good capacitive behavior. And the bulk solution resistance (Rs), derived from the intercept at the real axis, are 5.7, 6.3, and 8.0 Ω for N400, N500 and N600, respectively. In the high–frequency range, the semicircle diameter represents the charge transport resistance (Rct) which are determined to be 0.52, 1.48, 1.12 Ω, respectively, indicating that N400 electrode has the lowest charge transport resistance (Rct). As we
ACCEPTED MANUSCRIPT all know that the efficiency of ion diffusion and charge transport are limiting factors for the specific capacity of the supercapacitor. It can be concluded that the low charge transfer resistance results in the high specific capacity of N400 electrode. In addition, the cyclic stability is a significant criterion for supercapacitors. The cycling
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performance of N400, N500 and N600 electrodes were measured by continuous charge–discharge cycling at a current density of 5 A g–1 for 3000 cycles. As shown in Fig. 4, the corresponding capacitance retention of N400, N500, N600 are about 94%, 85% and 78%, respectively. The slight reduce of the specific capacitance of N400 may be due to a little dropping of active materials from
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the electrode. However, it is still better than other NiO materials reported previously [40–43]. Based on the above results, N400 is a desired electrode material for supercapacitor applications.
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To further investigate the practical application of NiO nanospheres, an asymmetric supercapacitor (ACS) based on N400 and active carbon (AC) materials was fabricated using CV and galvanostatic charge–discharge (GCD) measurements in 3 M KOH solution (Fig. 5). In addition, the electrochemical performance of AC electrode was also investigated using a three electrode system in 3 M KOH electrolyte (Fig. S6a–c). The specific capacitance of AC electrode is
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180 F g–1 at a current density of 1 A g–1, which was calculated from the discharge curves. It is significant to balance the charges stored at positive electrode (Q+) and negative electrode (Q–) for a supercapacitor with high operating voltage and high energy density [44]. The charges (Q) stored
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at each electrode can be calculated from the following eqn (3): Q = C × ∆V × m. To realize Q+ = Q–, the mass ratio between the N400 and AC electrodes can be determined by the eqn (4): m+ /
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m–= (C– × ∆V–) / (C+ × ∆V+). In this case, the optical mass ratio (m(N(400)/m(AC)) is about 0.8 for the ASC. The operating potential of the ASC was evaluated by using CV tests at the scan rate of 20 mV s–1 in different voltage windows, as shown in Fig. S6d. When the potential window is extended up to 1.5 V, the significant redox humps can be observed, indicating the pseudocapacitive properties of N400//AC ASC deriving from N400 electrode. After 1.5 V, there is a little distortion in the curve. Hence, the operating voltage window can be determined at 0–1.5 V. The CV curves of the N400//AC ASC at different scan rates from 5 to 50 mV s–1 are shown in Fig. 5a. With the increases of scan rate, the current density slowly increased with no significant distortion, indicating the great charge–discharge performance of the ASC device. Fig. 5b shows the GCD curves at different current densities ranging from 0.5 to 10 A g–1. The specific
ACCEPTED MANUSCRIPT capacitance of the ASC was calculated based on the total mass of active materials of each electrode (Fig. 5c). The highest specific capacitance is up to 68.5 F g–1 at 0.5 A g–1, which only declined to 40 F g–1 at a high current density of 10 A g–1. Moreover, no clear IR drop was found in the discharge curves, indicating the low internal resistance of the supercapacitor [45]. The cycling
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stability of the ASC was further evaluated at a current density of 5 A g–1 (Fig. 5d). Apparently, the ASC demonstrates admirable electrochemical cycling performance after 3000 cycles, and the capacitance retention can be maintained at 92.3%, indicating that there is a highly reversible redox reaction between the active materials and electrolyte.
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The power density and energy density are two key parameters to evaluate the energy storage performance of supercapacitors. The energy density (E) and power density (P) can be calculated
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by eqn (5) and eqn (6):
E=
1 / 2C∆V 2 (5) 3 .6
P=
3600 E ∆t
(6)
where E (Wh kg–1), P (W kg–1), C (F g–1), ∆V (V), ∆t (s) are the energy density, the power density,
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the specific capacitance, the range of the potential, and the total discharge time, respectively. To investigate the performance of the N400//AC ASC, a Ragone plot (the energy density vs power density) was carried as shown in Fig. 6. Obviously, the N400//AC ASC exhibits high energy
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density of 21.4 Wh kg–1 at a power density of 375.8 W kg–1, and still remains 12.5 Wh kg–1 even at a high power density of 7500 W kg–1, which outperforms many previously reported ASCs, such
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as NiO//Carbon (11.6 Wh kg–1 at 28 W kg–1) [46], NiCo2O4–MnO2//AG (9.4 Wh kg–1 at 2500 W kg–1) [47], Graphene–NiCo2O4//AC (7.6 Wh kg–1 at 5600 W kg–1) [48], Ni–Co oxide//AC (7.4 Wh kg–1 at 1903 W kg–1) [49], and MnO2 nanowire/Graphene//Graphene (7.0 Wh kg–1 at 5000 W kg–1) [50]. All the experimental results vividly indicate that the N400 hollow double–shelled nanospheres possess a potential application for high–performance supercapacitor.
4. Conclusions In conclusion, hollow core–in–double–shell NiO nanospheres have been successfully fabricated by the calcination of the Ni–based MOF precursors. As–synthesized NiO nanospheres
ACCEPTED MANUSCRIPT calcined at different temperatures have distinct surface areas and electrical conductivities, which play vital roles to the redox reaction occurred at the surface of the active materials. By contrast, N400 shows a specific capacitance of 473 F g–1 at the current density of 0.5 A g–1 and manifests a good retention after 3000 cycling tests. Meanwhile, the N400//AC ASC was assembled to further
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confirmed the potential application of the N400, which presents high energy storage capacity and exhibits excellent cycling performance. These results indicate that N400 is a promising electroactive candidate for supercapacitors. Furthermore, the MOF–derived metal oxides possess highly uniform porosity, which plays a crucial role in achieving the increases of electrochemical
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performance. Hence, the development of MOF–derived metal oxides possessing high surface area
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and well–defined shape is a promising strategy for surpercapacitor materials.
Acknowledgements
This work was supported by the National Natural Science Foundation of China (No. 21471086; No. 51572272), the Science and Technology Department of Zhejiang Province (No. 2017C33007), the Natural Science Foundation of Ningbo (No. 2017A610062; No. 2017A610065), and the K.C.
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Wong Magna Fund in Ningbo University.
Appendix A. Supplementary data
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Supplementary data associated with this article can be found, in the online version, at
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http://dx.doi.org/xxx.
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Fig. 1 (a) SEM image of MOF precursor. (b) TEM image of MOF precursor. (c,d) SEM images of N400. (e,f) TEM images of N400 with hollow core–in–double–shell nanospheres.
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Fig. 2 (a) XRD patterns of as–synthesized NiO nanospheres. (b–d) Nitrogen adsorption/desorption isotherms measured at 77 K for N400, N500, and N600, respectively. The insets show the
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corresponding BJH pore size distributions.
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Fig. 3 (a) CV curves of N400, N500, and N600 at a scan rate of 20 mV s–1. (b) CV curves of N400 at different scan rates. (c) GCD curves of N400, N500, and N600 at 0.5 A g–1. (d) GCD curves of N400 at different current densities. (e) Specific capacitance of N400, N500, and N600 at various current densities. (f) EIS curves of N400, N500, and N600 at room temperature in 3.0 M KOH solution.
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Fig. 4 Specific capacitance retention with the number of charging–discharging cycling test at a
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current density of 5 A g–1.
Fig. 5 (a) CV curves of the N400//AC ASC at different scan rates. (b) GCD curves of the
N400//AC ASC at various current densities. (c) The specific capacitances of the N400//AC ASC at different current densities. (d) Cycling performance of N400//AC ASC at a current density of 5 A g–1.
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Fig. 6 Ragone plots of the as–prepared N400//AC ASC compared to the reference.
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1. Hollow double-shelled NiO nanospheres were prepared from MOF precursors. 2. The NiO nanospheres show high specific capacitance.
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3. The NiO nanospheres exhibit excellent cycling performance.
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4. The assembled asymmetric supercapacitor presents high energy density.