Flower-shaped multiwalled carbon [email protected] acid MOF composite as a high-performance cathode material for energy storage

Electrochimica Acta 281 (2018) 69e77

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Flower-shaped multiwalled carbon nanotubes@nickel-trimesic acid MOF composite as a high-performance cathode material for energy storage Qinghua Wang a, Qingxiang Wang a, b, *, Biyan Xu a, Feng Gao a, Fei Gao a, Chuan Zhao b, ** a b

College of Chemistry and Environment, Minnan Normal University, Zhangzhou, 363000, PR China School of Chemistry, The University of New South Wales, Sydney, NSW, 2052, Australia

a r t i c l e i n f o

a b s t r a c t

Article history: Received 15 November 2017 Received in revised form 3 May 2018 Accepted 24 May 2018 Available online 25 May 2018

Metal-organic frameworks (MOFs) have received considerable attention in electrochemical fields due to their tunable morphology, open topological structure, large surface, and intense redox-activity. Herein, a novel flower-shaped multiwalled carbon nanotubes/nickel-trimesic acid composite (MWCNTs@Ni(TA)) was synthesized via one-step solvothermal method. The morphology and composition of the composite were characterized and compared with the single-component counterpart of spherical Ni(TA) by SEM, TEM, XRD, BET and FT-IR. The characterization results showed that the presence of carboxyl functionalized MWCNTs played a critical role to induce transformation of Ni(TA) from solid spherical shape in the single-component situation to the nanosheet-assembled flower shape in the composite. Electrochemical experiments showed that the flower-shaped MWCNTs@Ni(TA) composite have higher specific capacity (115 mAh g
1 at a current density of 2 A g
1) and better rate capability than the spherical Ni(TA). The MWCNTs@Ni(TA) based electrode also presented outstanding cycling stability with 81.6% specific capacity remained after 5000 charge-discharge cycles at a current density of 10 A g
1 in KOH electrolyte. © 2018 Elsevier Ltd. All rights reserved.

Keywords: Nickel-trimesic acid Multiwalled carbon nanotubes Nickel-trimesic acid Metal-organic frameworks Energy storage

1. Introduction As the quick progress of electric vehicle and portable electronic equipment in modern society, there are increasing needs for developing energy storage devices with both high energy and power densities. Nowadays, the energy storage devices mainly include superconducting magnetic energy storage coil [1], lithium batteries [2], fuel cell [3], supercapacitor [4], and etc. Among these various energy storage devices, supercapacitor has attracted much concern due to its quick charge-discharge process, excellent cycling stability, high coulombic efficiency, and outstanding environmentfriendliness [5e7]. Electrode materials with high power density and energy density play a key role in the construction of highperformance supercapacitors [8]. At present, there are three broad categories of electrode

* Corresponding author. College of Chemistry and Environment, Minnan Normal University, Zhangzhou, 363000, PR China. ** Corresponding author. School of Chemistry, The University of New South Wales, Sydney, NSW, 2052, Australia. E-mail addresses: [email protected] (Q. Wang), [email protected] (C. Zhao). https://doi.org/10.1016/j.electacta.2018.05.159 0013-4686/© 2018 Elsevier Ltd. All rights reserved.

materials for supercapacitor, including carbon [9,10], metal oxide/ hydroxide [11,12] and conductive polymers [13,14]. Carbonaceous materials have been widely used in commercial electrochemical double-layer supercapacitors due to their excellent stability. However, the low specific capacitance of the carbonaceous materials seriously limits their electrochemical performance. On the other hand, metal oxide such as RuO2 has higher specific capacitance than carbon, but the high price hampers its industrial application [15]. Alternatively, some non-precious transition metal oxide such as NiO [16] and Co3O4 [17] have been developed due to their good electrochemical behaviors, easy preparation and low cost. Nevertheless, the practical application of this kind of materials is also limited by their poor cycle stability. As the third kind of electrode material, conductive polymers have wide working voltage window, but their large volume contraction and expansion during the charge-discharge process can cause structural instability and fast capacitance recession during long-term application. Therefore, it is still a great challenge to seek novel high capacitance, low cost and robust electrode materials for the fabrication of the highperformance supercapacitors. Metal-organic frameworks (MOFs) are a type of porous material formed by transition metal ions linked with oxygen or nitrogen

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containing organic ligands [18,19]. MOFs have the features of large specific surface area, adjustable pore size, and versatile structures. Based on these merits, MOFs have been used in the area of catalysis [20,21], gas adsorption [22], sensing [23,24], etc. In addition, MOFs present good electrochemical activities through choosing appropriate metal centers or bridging ligands, which enable them owning great potential in the applications of electrochemistry. In addition, MOFs can provide more electrochemical active sites and promote the ions diffusion through controlling the structure and pore size. Thus, MOFs are recently used as a new type of outstanding electrode materials of supercapacitors [25e29]. However, the poor conductivity and inferior stability of MOFs result in the fact that the supercapacitors constructed by pure MOFs generally exhibit poor rate capability and cycle stability, which limit their wide applications as electrode materials in supercapacitors. To overcome these shortcomings of MOFs, the materials with high conductivity and mechanical stability have been utilized to form composites with MOFs. As a type of tube-structured carbon material, carbon nanotubes (CNTs) possess excellent conductivity, outstanding chemical stability, high mechanical strength, and good flexibility [30]. So far, CNTs were frequently used as substrates for the growth and fixation of active materials in energy storage devices, which enhances both the conductivity and stability of the electrode materials [31,32]. Thus, the decoration of porous MOFs with highly conductive CNTs should also be an effective strategy to enhance the electrochemical performance of the MOFs materials. For example, Zhang et al. synthesized a composite material of CNTs and Mn-MOF and used it as the electrode material to construct a symmetric supercapacitor [31]. The result displays that the hybridization of CNTs with the Mn-MOF result in an inherent improvement of electrochemical conductivity and specific capacitance (from 43 F g
1 for pure Mn-MOF to 203 F g
1 for the composite). Wen et al. have also prepared nickel-dicarboxybenzene MOF/CNTs (Ni-MOF/CNTs) composite-based electrode material [32]. Electrochemical results demonstrated that outstanding electronic conductivity of CNTs can provide small charge diffusion resistance and decreased path of electron collection/transport. All these studies indicate the hybridization of MOFs and CNTs can effectively improve the electrochemical performance of the materials with respect to single component. In this paper, the novel nickel-based MOF, Ni2þ-trimesic acid (Ni(TA)) and its composite with multiwalled carbon nanotubes were prepared by a facile solvothermal method. The results displayed that the CNTs can act as the backbone for the in-situ formation of the Ni-MOF nanosheets, and effectively trigger the

transformation of solid spherical Ni(TA) to the core-shell structured flower-like composite, MWCNTs@Ni(TA) (Scheme 1), through which the surface area and the electroactivity of the material was greatly improved. In addition, the capacitive behaviors of the synthesized materials were investigated and compared by cyclic voltammetry (CV), galvanostatic charge-discharge test (GCD), and electrochemical impedance spectroscopy (EIS). The results showed that the flower-shaped MWCNTs@Ni(TA) composite has higher specific capacity, better rate capability and superior cycling stability than the spherical Ni(TA), due to the synergetic effect of the nanosheet-structured Ni(TA) and the highly conductive MWCNTs. 2. Experimental 2.1. Apparatus and reagent The powder X-ray diffraction data (XRD) of the samples were recorded on a Bruker D8 Advance powder diffractometer (Germany) using Cu Ka radiation (l ¼ 1.54056 Å). The morphology was analyzed on an FEI Tecnai G2 F20 transmission electron microscope (TEM, USA) and a Hitachi SU8020 scanning electron microscope (SEM, Japan) with an X-ray energy dispersive spectrometer (EDS). The infrared spectrum was tested on a Nicolet 750 FT-IR spectrometer (USA). The N2 adsorptionedesorption isotherms were measured using a Quantachrome Autosorb-IQ automated gas sorption analyzer (USA) at 77 K. The electrochemical measurements were carried out on a Chenhua CHI660 electrochemical workstation (China). The cycling stability tests were performed on a Land CT2001A battery test system (China). Multiwalled carbon nanotubes (MWCNTs) were provided by Shenzhen Nanotech Port Corporation (China), and acidified to form functional carboxyl group according to literature [31] before use. Ni(NO3)2$6H2O, polyvinylpyrrolidone (PVP) and KOH were bought from Xilong reagent Inc. Corp. (China), trimesic acid (TA) and polytetrafluoroethylene (PTFE) were provided by Aladdin Reagent Inc. Corporation (China). The water used throughout the experiments was ultra-pure water purified by a Millipore-Q system. 2.2. Hydrothermal synthesis of MWCNTs@Ni(TA) composite Typically, 70 mg of carboxyl functionalized MWCNTs was added into 70 mL of methanol and dispersed with ultrasonication until homogeneous black solution was formed. Then 250 mg of PVP, 0.637 g (2.2 mmol) of Ni(NO3)2$6H2O and 0.256 g (1.2 mmol) of TA were added to the above MWCNTs dispersion. After further stirring

Scheme 1. Schematic illustration of the synthesis process of Ni(TA) and MWCNTs@Ni(TA)

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for 60 min at room temperature, the mixture was transferred into a 100 mL autoclave and kept at 150  C for 24 h. After cooling down to ambient temperature naturally, the product was collected by centrifugation and washing with methanol for 3 times, and finally dried under 60  C for 12 h to get the MWCNTs@Ni(TA) composite. To study the influence of amount of MWCNTs on the electrochemical behaviors of the composite, the control samples with various amount of MWCNTs (0, 30, and 100 mg) added were also prepared via the similar route.

2.3. Electrochemical characterization Electrochemical tests were carried out in a 2 M KOH solution with a three-electrode system, which consisted of an active materials modified working electrode, a Pt wire counter electrode and an Ag/AgCl (3 M KCl) reference electrode. The working electrodes were prepared by mixing the synthesized active material, carbon black and PTFE homogeneously in ethanol by an agate mortar with a mass ratio of 8:1:1, and then coating the mixture on nickel foams (1  1 cm2). After dried at 80  C in vacuum for 12 h, the nickel foams were pressed under 10 MPa for 10 s. The loading mass of the active materials was calculated to be about 2 mg cm
2. The specific capacity of the electrode material is calculated by equation (1) from galvanostatic charge-discharge (GCD) method.

Cs ¼

I$Dt 3600$m

(1)

In equation (1), the symbols of Cs, I, Dt and m refer to the specific capacity (mA h g
1), current (mA), discharge time (s) and mass of active materials (g), respectively.

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3. Results and discussion 3.1. Structure and morphology characterization The SEM and TEM images of Ni(TA) and MWCNTs@Ni(TA) are shown in Fig. 1. As seen, the synthesized Ni(TA) are microsphere structured with the diameter in the range from 2 to 5 mm (Fig. 1a). The SEM image of a single Ni(TA) microsphere shows that the spherical Ni(TA) has smooth surface (Fig. 1b). TEM image as showed in Fig. 1c proves that the Ni(TA) materials are solid spheres. Such a thick solid sphere morphology will prevent the diffusion of electrolyte ions into the interior of the material, and only the surface area of the spheres is accessible by the ions, resulting in low capacitive behaviors, as will be discussed later. On contrary, when MWCNTs were added during the synthesis of Ni(TA), it was found that the sphere structure was totally absent in the product, instead lots of flower-shaped clusters with vertically aligned thin sheets were formed (Fig. 1d and e). The sheets to constitute the nanoflower have the thickness of only about 13 nm in average. In addition, the adjacent nanoflowers clusters have the macropores with the width of about 200 nm, and the adjacent nanosheets have the gap with the distance of about 23 nm. These structure characters mean that the synthesized nanoflowers have large surface area and meanwhile the ions can easily diffuse between the sheets along the macro/mesopores. From the TEM image shown in Fig. 1f, the nanoflowers clusters are observed to be porous in nature. Furthermore, from a higher magnification TEM image shown in Fig. 1g, one can see that the MWCNTs are uniformly coated with the Ni(TA) nanosheets, forming a core-shell structure. Fig. 1h displays the HRTEM image of the MWCNTs@Ni(TA) composite. The lattice fringes with a distance of 0.34 nm agree well with the inner-walls separation of MWCNTs. In addition, there are lots of

Fig. 1. SEM (a and b) and TEM images (c) of Ni(TA). SEM (d and e), TEM (f and g), HRTEM images (h) and EDS spectra (i) of MWCNTs@Ni(TA) composite. The arrows in images d and f point to the macropores. The red arrows in image g point to a MWCNT with diameter of about 60 nm. The black arrows in image h point to the micro/mesopores structure of the composite. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)

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micro- and mesopores with diameters of about 1e3 nm in the MWCNTs@Ni(TA) composite (marked by black arrows in Fig. 1h). The EDS analysis (Fig. 1i) clearly shows the presence of only Ni, O and C elements in the MWCNTs@Ni(TA) composite, with an atomic percentage of 5.90% for Ni. In summary, the morphological characterization results showed that the Ni(TA) nanosheets are interconnected by the flexible MWCNTs, which will enhance the mechanical strength of the material and prevent collapse of the structure during the chargingdischarging process. The hierarchical porous structure will also improve the specific surface area of the composite material, increase the contact area between the electrolyte and electrode material, and reduce the distance of charge transfer. The FT-IR spectra of the ligand TA, the synthesized Ni(TA) and MWCNTs@Ni(TA) were presented in Fig. 2a. For the free TA ligand, the Ar-COOH vibration bands are observed at 3000e2500 cm
1. The C¼O and C
O vibration bands are observed at 1720 and 1275 cm
1, respectively. Upon complexation of TA with Ni2þ ions, the vibration bands at 3000e2500 cm
1 and 1720 cm
1 disappeared, confirming that the carboxyl groups of the TA ligands have been deprotonated. The new peaks appeared at 1619 cm
1 and 1369 cm
1 are related to the asymmetric stretching and symmetric stretching vibrations of -COOH, respectively [31]. The peak at 1288 cm
1 is ascribed to the stretching vibration of C
O bond in the uncoordinated free carboxyl on the surface of the MOF. The peaks at 761 and 722 cm
1 are caused by the out-of-plane bending vibration of C-H, and the peak at 1557 cm
1 is assigned to the stretching vibration of aromatic C¼C. For the MWCNTs@Ni(TA) composite, it is found that all the main peaks appeared in Ni(TA) remained, suggesting that the fundamental chemical structure of the Ni(TA) MOFs have not been changed during synthesis of the

composite. In addition, the nC
O peak at 1288 cm
1 disappeared, which may be caused by the formation of hydrogen bonds between the free carboxyl on the MOFs with the derivated oxygencontaining groups from MWCNTs. The crystallographic structure of Ni(TA) and MWCNTs@Ni(TA) were analyzed by XRD (Fig. 2b). As shown in Fig. 2b, the diffraction pattern of Ni(TA) contains a broad hump at 10.5 , which can be ascribed to the (111) diffraction of Ni(TA). The low intensity of other diffraction peaks and the relatively high background indicate poor crystallinity of the synthesized Ni(TA) [33]. For the composite, the diffraction pattern shows some differences compared with that of Ni(TA). First, a new peak appeared at 26.6 , which could be attributed to the (002) diffraction of the MWCNTs. In addition, two new weak peaks at 28.1 and 32.4 appeared, which can be attributed to the (11‾3) and (12‾3) diffraction of Ni(TA) [34]. These results also suggested that the presence of MWCNTs during the synthesis process can improve the crystallinity of the Ni-MOF. The specific surface area and pore-structure are two critical factors that affect the capacitive property of supercapacitors. So, the nitrogen adsorption-desorption experiment was further applied to determine the BET surface area and pore feature of the samples. Fig. 2c shows the typical N2 adsorption-desorption curves of Ni(TA) and MWCNTs@Ni(TA). As seen, the shape of the nitrogen adsorption-desorption isotherms revealed that the pure Ni(TA) is not mesoporous, with a low calculated BET surface area of 20.0 m2 g
1. On the contrast, the MWCNTs@Ni(TA) displayed a typical type IV adsorption isotherm with a H1-type hysteresis loop at P/Po ¼ 0.45e0.9, which indicated the presence of mesoporous structure for the composite. The calculated BET surface area for MWCNTs@Ni(TA) is 85.5 m2 g
1. The pore size distribution for MWCNTs@Ni(TA) composite was further achieved using the

Fig. 2. FI-IR spectra of TA, Ni(TA) and MWCNTs@Ni(TA) (a). XRD patterns (b) and N2 adsorption-desorption isotherms (c) of Ni(TA) and MWCNTs@Ni(TA). BJH desorption pore size distribution of MWCNTs@Ni(TA) (d). Inset of (d) is the magnification of distribution curve in the pore diameter range from 2 to 5 nm. The samples were pre-activated at 120  C for 10 h under vacuum before the N2 adsorption-desorption test.

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BarreteJoynereHalenda (BJH) model on basis on the desorption branch and the result was displayed n in Fig. 2d. Clearly, a sharp peak centered at 3.5 nm is appeared in the pore size distribution plot, which agrees well with the mesopore characteristic as shown in the HRTEM image (Fig. 1h). Besides the sharp peak, also a wide peak is located at 10e20 nm, which is likely originated from the gap between adjacent nanosheets as shown in SEM image (Fig. 1e). These differences of BET surface area and pore size data between Ni(TA) and MWCNTs@Ni(TA) are in good consistence with the aforementioned morphological variation as showed in Fig. 1, which confirms that the presence of MWCNTs plays an important role to regulate the physical characteristic of Ni(TA). 3.2. Electrochemical evaluation of MWCNTs@Ni(TA) To explore the potential application of the synthesized MWCNTs@Ni(TA), the material was utilized as electrode materials for supercapacitor and its pseudocapacitive properties were investigated by CV, GCD and EIS using a three electrodes system in 2 M KOH aqueous solution. To obtain the optimum ratio of MWCNTs to Ni(TA), various amount of MWCNTs were added during the hydrothermal synthesis as described in the experimental section. The specific capacities of the obtained composite were calculated according to GCD test, with the results summarized in Fig. S1. As seen, the MWCNTs@Ni(TA) with 70 mg of MWCNTs added has a large capacity of 115 mA h g
1 at a current density of 2 A g
1, which is much larger than that of control samples with 0, 30 or 100 mg of MWCNTs. Hence, 70 mg was chose as the best amount of MWCNTs for the synthesis of the high performance capacitor electrode material. The CVs of MWCNTs@Ni(TA) and Ni(TA) at various scan rates were carried out, and the results are shown in Fig. 3a and Fig. S2,

73

respectively. As shown in the two figures, a pair of redox peaks is visible in each voltammogram at different scan rate, suggesting that the measured capacity of both Ni(TA) and MWCNTs@Ni(TA) are mainly based on the redox mechanism, which corresponds to the electron transfer of Ni(III)/Ni(II) couple in active centers of the Ni(TA) MOFs. The peak currents enhanced with the increment of the scan rate, and the anode peak current is approximately equal to the cathode peak current, indicating that the Faradic reactions of the electrode are reversible. In addition, as the scan rate increase, the anodic peaks and cathodic peaks shifted to higher potentials and lower potentials, respectively, this can be explained by the polarization effect of the electrode material. Furthermore, for both MWCNTs@Ni(TA) (Fig. 3b) and Ni(TA) (Fig. S3) electrodes, the obtained peak current densities correlated well to the square root of the scan rate (n1/2) with a linear relationship within the scan rates from 5 to 50 mV s
1, indicating that the electrochemical process of Ni(TA) and MWCNTs@Ni(TA) are both controlled by the diffusion of OH
from electrolyte to electrode surface. Thus, the reaction mechanism of the synthesized Ni(TA) MOF under alkaline condition can be deduced as the following two steps (Scheme 2): The NiO6 unit in Ni(TA) MOF was first partly transferred to Ni(OH)2 via chemical reaction with OH
under alkaline condition; then the formed Ni(OH)2 was electrochemically oxidized to NiOOH via an one-electron coupled with one OH
process. The CV curves of Ni(TA) and MWCNTs@Ni(TA) at a fixed scan rate of 30 mV s
1 were compared in Fig. 3c. As seen, the flowerstructured MWCNTs@Ni(TA) has much higher redox peak currents and larger integrated area than that of Ni(TA), indicating that MWCNTs@Ni(TA) has better pseudocapacitive properties than Ni(TA). Moreover, according to the reduction peak in CV curve and the Faraday's law [35]:

Fig. 3. (a) CV curves of MWCNTs@Ni(TA) in 2 M KOH at different scan rates. (b) The linear relationships between the anodic/cathodic peak currents and the scan rates for MWCNTs@Ni(TA). (c) CV curves of Ni(TA) and MWCNTs@Ni(TA) at a scan rate of 30 mV s
1.

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Q. Wang et al. / Electrochimica Acta 281 (2018) 69e77

O

O

O O O

OH

O Ni H O

O

+2OH-

OH

O Ni HO

O O

O

+OH- -e-

Ni O

O

O

O O O

+ 2H2O

OH

O

O

+ H2O

Scheme 2. Proposed electrochemical reaction mechanism of the Ni(TA) MOFs.

Q ¼ nFAG*

(2)

where Q is the total charge amount for the reduction reaction, n the electrons-transferred number, F the Faraday's constant, and A the geometric area of nickel foam (1 cm2), the density (G*, mol cm
2) of the electroactive centers on Ni(TA) and MWCNTs@Ni(TA) modified electrodes were estimated to be 2.1 and 3.3 mmol cm
2, respectively, which further demonstrated that the effective electroactive centers are enlarged through using MWCNTs as the electron transfer channels and morphological agents. The GCD curves at a current density of 4 A g
1 as shown in Fig. 4a further reveal that the discharging time of MWCNTs@Ni(TA) is 92 s, which is much longer than that of Ni(TA) (75 s), proving the better capacitive property of MWCNTs@Ni(TA). The GCD curves of MWCNTs@Ni(TA) and Ni(TA) between 0 and 0.45 V at different current densities were tested and the results are shown in Fig. 4b

and Fig. S4, respectively. The potential plateaus in the charge and discharge curves caused by the redox reaction on the surface of the electrode are quite different from the triangular shape of doublelayer capacity, confirming the pseudocapacitive natures of the electrode materials. The GCD curves are nearly symmetrical, demonstrating good reversibility of the electroactive materials during charge-discharge cycles. The specific capacities of the two electrode materials at various current densities were calculated according to equation (1) and the results are shown in Fig. 4c. For Ni(TA) and MWCNTs@Ni(TA), the specific capacities at a current density of 2 A g
1 are 94 and 115 mA h g
1, respectively. When the current density is increased to 10 A g
1, the specific capacities decreased to 69 and 89 mA h g
1, with rate capability of 73.3% and 77.6%, respectively. These results testified that the presence of MWCNTs enhanced the specific capacity and rate capability of the electrode, which could be explained by the increase of both specific surface area and conductivity of the hierarchical flower-like

Fig. 4. GCD curves for Ni(TA) and MWCNTs@Ni(TA) at a fixed current density of 4 A g
1 (a). GCD curves of MWCNTs@Ni(TA) at different current densities. (b). Specific capacity versus different current densities (c) and cycling stability performance at a current density of 10 A g
1 (d) for Ni(TA) and MWCNTs@Ni(TA).

Q. Wang et al. / Electrochimica Acta 281 (2018) 69e77

composite relative to the solid spherical Ni(TA). Furthermore, the specific capacity of the MWCNTs@Ni(TA) composite is compared with MWCNT and other MOFs-based materials reported previously (Table S1). The result shows that the synthesized MWCNTs@Ni(TA) composite has a high specific capacity of 115 mA h g
1, which is obviously higher than that of MWCNTs (14 mA h g
1 at 0.4 A g
1) [30], Ni(TA) (94 mA h g
1 at 2 A g
1), and many other MOFs, such as Ni-MOF12 (49 mA h g
1 at 2 A g
1) [29], Ni-MOF (81 mA h g
1 at 1 A/g) [36], Ni-Co MOF (93 mA h g
1 at 2 A g
1) [37], Co-MOF (29 mA h g
1 at 0.6 A/g) [38], Co-BPDC MOF (27 mA h g
1 at 10 mV s
1) [39], Zr-MOF (Uio-66) (69 mA h g
1 at 20 mV s
1) [40], Ni-DMOF-ADC (69 mA h g
1 at 1 A g
1) [41], and so on. The superior performance of the MWCNTs@Ni(TA) composite could be ascribed to unique nanosheet-assembled flower shape of the composite with large surface area and the synergistic effect between the MWCNTs and Ni(TA), that is, Ni(TA) acts as the capacitive center with rich redoxactive sites and MWCNTs act as the outstanding pathway for electron transfer of Ni(TA). The cycling performance is another important factor to evaluate the performance of supercapacitors. Herein, the cycling stability of the electrode materials is tested by GCD measurements for 5000 cycles at a fixed current density of 10 A g
1 (Fig. 4d). The first and last ten cycles of GCD curves of Ni(TA) and MWCNTs@Ni(TA) are shown in Fig. S5. The results show that the shapes of chargedischarge curves of MWCNTs@Ni(TA) remained nearly unchanged after 5000 cycles, demonstrating that the electrode materials maintained the charge storage property during the cycling tests. The specific capacity of MWCNTs@Ni(TA) decreased from 89 to

75

73 mA h g
1 after 5000 cycles, remaining as high as 81.6% of the initial value. However, the specific capacity of Ni(TA) decreases from 69 to 32 mA h g
1, remaining only 45.4% of the initial value. From this comparison result, it can be obtained that upon the decorating of Ni(TA) with MWCNTs, the cycling stability was improved remarkably. This can be attributed to the fact that the good mechanical flexibility of MWCNTs in the composite was served as a mechanical buffer to accommodate the volume change during the charging/discharging process. Internal resistance (Rs) of an energy storage device, either a battery or a supercapacitor, is an important parameter that determines their electrochemical performance. The Rs values of Ni(TA) and MWCNTs@Ni(TA) modified electrodes were determined from the initial voltage drops (IR drops) of the discharge curves as defined in Fig. 4b. Then, the dependence of IR drop on the applied currents was fitted to linear plots and the results were shown in Fig. 5a. As seen, the linear fitting plot of MWCNTs@Ni(TA) presents a smaller slope than that of Ni(TA). Then according to the slopes, the Rs values (¼ 0.5  slope) was estimated to be 0.66 and 0.85 U for MWCNTs@Ni(TA) and Ni(TA), respectively. The smaller internal resistance for the composite electrode also facilitates high discharge power delivery in practical applications. The electron transportation and ions diffusion at the electrode/ electrolyte interface are two critical parameters to decide the cycling stability and rate capability of supercapacitors. So, the EIS measurements were conducted to explore the charge transport properties. From the result shown in Fig. 5b, it is found that the Nyquist curve of MWCNTs@Ni(TA) presents a straight line with a larger slope than that of Ni(TA) in the low-frequency region, which

Fig. 5. (a) IR drops as a function of current. (b) Nyquist impedance spectra of Ni(TA) and MWCNTs@Ni(TA). Inset is the magnification of the high frequency region of the spectra. (c) Plots of Zʹ against u
1/2 obtained from the EIS test. (d) Schematic diagram showing the charge transfer route in the MWCNTs@Ni(TA) electrode.

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demonstrated a smaller diffusion resistance (Warburg resistance) of ions from the electrolyte solution to electrode surface. And this result testifies the conclusion achieved from the slope difference of peak current density versus v1/2 in CV test (Fig. 3b). Furthermore, one can see that the MWCNTs@Ni(TA) electrode has smaller semicircle diameter in the high-frequency region than Ni(TA), indicating a lower charge transfer resistance (RCT) at the electrodeelectrolyte interface. From the magnification image of the high frequency region (inset of Fig. 5b) of the Nyquist curves, one can more clearly see that the MWCNTs@Ni(TA) electrode has a smaller intercept at the real axis than that of Ni(TA), which confirmed that MWCNTs@Ni(TA) has a smaller internal resistance. And the internal resistance values were determined to be 0.51 and 0.68 U from the intercepts for MWCNTs@Ni(TA) and Ni(TA), respectively, which is very close to the aforementioned values calculated from the IR drops. Therefore, from the IR drops and EIS characterization, we can conclude that the MWCNTs@Ni(TA) electrode exhibit more favorable charge transfer kinetics than Ni(TA) due to the special structure and composition of the MWCNTs@Ni(TA) composite. In addition, the plots of Zʹ against u
1/2 were obtained from the EIS test and fitted to linear plots (Fig. 5c). The smaller slope (s) for Ni(TA)/MWCNTs composite shows the larger diffusion coefficient of OH
than Ni(TA). Furthermore, the ion diffusion coefficients (D) can be calculated according to equation (3) [42]: 2 2

D¼

R T

2A2 n4 F4 C2 s2

(3)

Here, R is the gas constant (8.314 J K
1 mol
1), T the room temperature (298.15 K), A the geometric surface area of the electrode (1 cm2), n the number of the electrons transferred (1), F the Faraday constant (96485 C mol
1), C the concentration of OH
(2 M), and s is the slope of the plot Zʹ versus u
1/2 based on Z0 ¼ Rs þ Rct þ su
1/2. According to equation (3), the OH
diffusion coefficients (D) for Ni(TA) and MWCNTs@Ni(TA) were estimated to be 0.573  10
10 and 1.48  10
10 cm2 s
1, respectively. The diffusion coefficients of OH
in MWCNTs@Ni(TA) is as large as three times of that in Ni(TA), which further demonstrated that the Ni(TA)/MWCNTs electrode exhibit more favorable ions transfer kinetics than Ni(TA). Fig. 5d illustrates the electrochemical reaction process of the MWCNTs@Ni(TA) when it was applied as a supercapacitor material, from which the better charge transfer kinetics and superior capacitive property of the synthesized MWCNTs@Ni(TA) composite could be attributed to the following reasons: (1) the MWCNTs act as a stable scaffolds for Ni(TA) nanosheets growth and formed a hierarchical structure with large surface area, which increased the availability of the active material; (2) the macro/meso porous feature formed from Ni(TA) nanosheets can promote the diffusion of electrolyte ions along the inter-sheets gaps; (3) the highly conductive MWCNTs in the core-shell structure effectively facilitate the transportation of electrons in the electrode material, resulting in the high electrochemical response. 4. Conclusions Spherical Ni(TA) MOFs and nanoflower shaped MWCNTs@Ni(TA) composite were synthesized through hydrothermal reaction, and their capacitive behaviors were studied. Our results demonstrated that the introduction of carboxyl functionalized MWCNTs in the solvothermal process changed the morphology of the MOFs from solid spheres to porous nanoflowers cluster. The resulted MWCNTs@Ni(TA) composite has the larger specific surface area, enhanced specific capacity, and better cycling stability than the Ni(TA) MOFs. The superior properties of the

MWCNTs@Ni(TA) composite can be attributed to its unique threedimensional porous nanoflowers structure and the good bonding between Ni(TA) and MWCNTs. These results demonstrate that the carboxyl functionalized MWCNTs can act as an effective morphology controlling template for the synthesizing flowershaped MOF composite, and their outstanding electronic conductivity can significantly promote capacitive behavior of the MOF material, which opens the way for the utilization of MOFs in the area of electrochemical energy storage. Acknowledgements The work is supported by National Natural Science Foundation of China (21275127), Australian Research Council (FT170100224), Oversea Visiting Program of China Scholarship Council (CSC, No. [2016]3035), and Education Science Research Project for Young and Middle-aged Teachers of Fujian (Nos. JA15305, JA15314). Appendix A. Supplementary data Supplementary data related to this article can be found at https://doi.org/10.1016/j.electacta.2018.05.159. References [1] G. Indira, T. UmaMaheswaraRao, S. Chandramohan, Enhancing the design of a superconducting coil for magnetic energy storage systems, Physica C: Supercond. Appl 508 (2015) 69. [2] N. Liu, Z.D. Lu, J. Zhao, M.T. McDowell, H.W. Lee, W. Zhao, Y. Cui, A pomegranate-inspired nanoscale design for large-volume-change lithium battery anodes, Nat. Nanotechnol. 9 (2014) 187. [3] X. Xie, M. Ye, L.B. Hu, N. Liu, J.R. McDonough, W. Chen, Y. Cui, Carbon nanotube-coated macroporous sponge for microbial fuel cell electrodes, Energy Environ. Sci. 5 (2012) 5265. [4] M. Beidaghi, Y. Gogotsi, Capacitive energy storage in micro-scale devices: recent advances in design and fabrication of micro-supercapacitors, Energy Environ. Sci. 7 (2014) 867. [5] J.R. Miller, P. Simon, Electrochemical capacitors for energy management, Science 321 (2008) 651. [6] P. Simon, Y. Gogotsi, B. Dunn, Where do batteries end and supercapacitors begin, Science 343 (2014) 1210. [7] G. Shekhar, P.B. Karandikar, M. Rai, Investigation of carbon material derived from leaves of tree for the electrodes of supercapacitor, Inter. J. Emerging Eng. Res. Tech. 2 (2014) 127. [8] P. Simon, Y. Gogotsi, Materials for electrochemical capacitors, Nat. Mater. 7 (2008) 845. [9] E. Frackowiak, Carbon materials for supercapacitor application, Phys. Chem. Chem. Phys. 9 (2007) 1774. [10] J. Zhi, W. Zhao, X.Y. Liu, A. Chen, Z. Liu, F. Huang, Highly conductive ordered mesoporous carbon based electrodes decorated by 3D graphene and 1D silver nanowire for flexible supercapacitor, Adv. Funct. Mater. 24 (2014) 2013. [11] J. Jiang, Y. Li, J. Liu, X. Huang, C. Yuan, X.W.D. Lou, Recent advances in metal oxide-based electrode architecture design for electrochemical energy storage, Adv. Mater. 24 (2012) 5166. [12] W.P. Sun, X.H. Rui, M. Ulaganathan, S. Madhavi, Q. Yan, Few-layered Ni(OH)2 nanosheets for high-performance supercapacitors, J. Power Sources 295 (2015) 323e328. [13] Y. Zhao, B.R. Liu, L.J. Pan, G. Yu, 3D nanostructured conductive polymer hydrogels for high-performance electrochemical devices, Energy Environ. Sci. 6 (2013) 2856. [14] T.M. Higgins, J.N. Coleman, Avoiding resistance limitations in highperformance transparent supercapacitor electrodes based on large-area, high-conductivity PEDOT: PSS films, ACS Appl. Mater. Interfaces 7 (2015) 16495. [15] R.R. Bi, X.L. Wu, F.F. Cao, L.Y. Jiang, Y.G. Guo, L.J. Wan, Highly dispersed RuO2 nanoparticles on carbon nanotubes: facile synthesis and enhanced supercapacitance performance, J. Phys. Chem. C 114 (2010) 2448. [16] S.I. Kim, J.S. Lee, H.J. Ahn, H.K. Song, J.H. Jang, Facile route to an efficient NiO supercapacitor with a three-dimensional nanonetwork morphology, ACS Appl. Mater. Interfaces 5 (2013) 1596. [17] X.H. Xia, J.P. Tu, Y.J. Mai, X.L. Wang, C.D. Gu, X.B. Zhao, Self-supported hydrothermal synthesized hollow Co3O4 nanowire arrays with high supercapacitor capacitance, J. Mater. Chem. 21 (2011) 9319. [18] N. Stock, S. Biswas, Synthesis of metal-organic frameworks (MOFs): routes to various MOF topologies, morphologies, and composites, Chem. Rev. 112 (2011) 933. [19] X.Y. Liu, W.J. Gao, P.P. Sun, Z. Su, S. Chen, Q. Wei, S. Gao, Environmentally

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