NiCo-MOF nanosheets wrapping polypyrrole nanotubes for high-performance supercapacitors

Journal Pre-proofs Full Length Article NiCo-MOF nanosheets wrapping polypyrrole nanotubes for high-performance supercapacitors Yuexin Liu, Yanzhong Wang, Yanjun Chen, Chao Wang, Li Guo PII: DOI: Reference:

S0169-4332(19)33906-6 https://doi.org/10.1016/j.apsusc.2019.145089 APSUSC 145089

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Applied Surface Science

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28 September 2019 4 December 2019 14 December 2019

Please cite this article as: Y. Liu, Y. Wang, Y. Chen, C. Wang, L. Guo, NiCo-MOF nanosheets wrapping polypyrrole nanotubes for high-performance supercapacitors, Applied Surface Science (2019), doi: https:// doi.org/10.1016/j.apsusc.2019.145089

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NiCo-MOF nanosheets wrapping polypyrrole nanotubes for highperformance supercapacitors Yuexin Liua, Yanzhong Wanga,b*, Yanjun Chena,b, Chao Wanga,b, Li Guob* aSchool

of Materials Science and Engineering, North University of China, Taiyuan 030051, P. R. China

bAdvanced

energy materials and system institute, North University of China, Taiyuan 030051, P. R. China

Abstract: Metal-organic frameworks (MOFs) are widely used as supercapacitor electrode materials because of their large specific surface area and tunable pore structure. However, the limited electronic conductivity of MOFs deteriorates the capacitive performance. Herein, bimetal-organic framework nanosheets wrapping polypyrrole nanotubes (NiCo-MOF@PNTs) were successfully prepared by an easy ultrasonic method. In the NiCo-MOF@PNTs nanocomposites, PNTs can boost the electronic conductivity of NiCo-MOF and effectively impede the aggregation of NiCoMOF nanosheets. The unique structure of NiCo-MOF@PNTs allows for exhibiting the superior electrochemical performance. The specific capacitance of NiCo-MOF@PNTs is 1109 F g-1 at 0.5 A g-1. Furthermore, the assembled NiCo-MOF@PNTs//AC asymmetric supercapacitor demonstrates a high energy density of 41.2 Wh kg-1 at a power density of 375W kg-1, and the excellent cycling stability with a capacitance

*

Corresponding author: Tel. / Fax: +86-351-3557519. E-mail: [email protected] (Y.Z. Wang), and [email protected] (L. Guo). 1

retention of 79.1% after 10000 cycles, demonstrating that NiCo-MOF@PNTs nanocomposites are a promising candidate for supercapacitor electrode materials. Keywords: MOF; Polypyrrole nanotubes; Asymmetrical supercapacitor

1 Introduction With a large number of fossil fuel consumption, the environmental protection is increasingly urgent, which stimulates researchers to develop a clean and sustainable energy, such as solar energy, wind energy, and geothermal energy [1-2]. However, these renewable energies are heavily dependent on the environment, and they are intermittent. Therefore, efficient energy storage devices are crucial for utilizing these new energy sources. Many energy storage devices such as batteries, fuel cells and capacitors, have been widely investigated [3-4]. Supercapacitors demonstrate a great potential as a new excellent energy storage device owing to their fast charge-discharge rate, wide operating temperature range, and long-term stability [5]. It is well known that the performance of supercapacitors depends mainly on electrode materials [6]. The superpcapacitor electrode materials are mainly divided into three types: carbon-based materials, conductive polymers and metal compounds [7-9]. Carbon materials are often used as a negative electrode due to their large specific surface area, high electronic conductivity, and chemical stability, such as activated carbons, graphene, and carbon nanotubes. However, the carbon-based symmetrical supercapacitors have low specific capacitance and energy density [10-11]. Transition metal oxides are promising electrode materials because they have multiple oxidation states and high specific capacitance. However, the poor cycling stability limits their practical application for 2

high-performance supercapacitors [12, 13]. Metal-organic frameworks (MOFs) with regular tunable structure, ultra-high porosity, and large surface area have been widely used as multi-functioning materials in the field of electrochemistry [14-16], such as electrocatalysis [17], lithium sulfur batteries [18], lithium-ion batteries [19], capacitive deionization [20] and supercapacitors [21]. MOFs consist of transition metal ions and organic linkers, forming a mass of interpenetrating pore structures, which would facilitate the transfer of electron and ion [22-24]. Furthermore, the structure of MOFs is highly ordered and porous, providing more exposed redox active sites for energy storage. Previous reports showed that Co-MOFs have excellent reversibility with the low specific capacitance, while Ni-MOFs have high specific capacitance with the low reversibility [25]. Therefore, Co and Ni bimetalorganic frameworks would display better electrochemical properties than that of monometal-organic frameworks owing to the synergistic effect of cobalt and nickel ions in MOFs [26-28]. However, the poor electronic conductivity of MOFs becomes a big obstacle for their applications in supercapacitors [29-31]. In order to solve this problem, two feasible strategies have been proposed. One method is converting MOFs into porous carbon, metal oxides (sulfides, phosphides) or their composites via the pyrolysis at high temperature, delivering an excellent electrochemical performance. However, the high temperature treatment inevitably destroys the framework of MOFs, resulting in the reduction of the specific surface area and electroactive sites. Another solution is combining MOFs with conductive substrates, such as carbon fibers [32], CNTs [33, 34], graphene [35, 36], and conducting polymers 3

[37], which can enhance the electronic conductivity of MOFs and impede the aggregation of MOFs. For example, Tian’s group reported a feasible route to coat the pre-oxidized polyacrylonitrile nanofibers with Ni-MOF nanosheets, and the composites exhibited the specific capacitance of 702.8 F g-1 at 0.5 A g-1 [32]. Lan et al. designed the unique polyoxometalate-based MOFs with the conductive polypyrrole, which exhibited a high specific capacitance of 5147 mF cm-2 at a scan rate of 10 mV s-1 [38]. Recently, two dimensional (2D) MOF nanosheets have drawn intensive attention for high-performance supercapacitor owing to the large specific surface area [39, 40]. Meanwhile, the ultrathin MOF nanosheets can provide the short transfer pathway of ion and electron in the framework, resulting in the excellent rate capability. For instance, Wang et al. reported that the ultrathin NiCo-MOF nanosheets exhibit the excellent capacitive performance of 1206 F g-1 at 1 A g-1 [41]. Nevertheless, the ultrathin NiCoMOF nanosheets display low rate capability and cycling stability due to their relatively low electronic conductivity. Polypyrrole nanotubes (PNTs) are widely used as the conductive substrate to enhance the capacitive performance of transition metal oxide (hydroxide) [42-43], transition metal sulfide [44], and MOFs [5, 45] due to their excellent electronic conductivity, large specific surface area, and remarkable energy storage capacity/reversibility [46-48]. For instance,

Wang

et

al.

synthesized

Ni1/3Co2/3(CO3)0.5OH∙H2O@PNTs

and

NiCo2S4@PNTs, respectively, exhibiting the excellent capacitive performance [49, 50]. Herein, PNTs can boost the electronic conductivity as well as the uniform distribution of metal compounds. To our knowledge, the capacitive performance of ultrathin NiCo4

MOF nanosheets with PNTs was not reported. It is expected that PNTs would enhance the electronic conductivity of NiCo-MOF nanosheets, and impede their aggregations. Inspired by the above reports, we synthesized NiCo-MOF nanosheets wrapping PNTs via a simple ultrasonic method at room temperature. PNTs play multiple roles in improving the capacitive performance of NiCo-MOF: (i) Using as the substrate for in situ growth of NiCo-MOF nanosheets, (ii) Hindering the agglomeration of NiCo-MOF, and providing larger specific surface area and more active sites, (iii) Acting as the conductive network to enhance the electronic conductivity of NiCo-MOF nanosheets, and (iv) Providing extra pseudocapacitance [51]. These unique structures allow NiCoMOF@PNTs for delivering the outstanding capacitive performance.

2 Experimental 2.1 Chemicals CoCl2·6H2O, NiCl2·6H2O, FeCl3·6H2O, methyl orange (MO), p-phthalic acid (PTA), pyrrole, Triethylamine (TEA), N, N-dimethylformamide (DMF) and anhydrous ethanol was purchased from Sinopharm Chemical Reagent Co. Ltd, and used without further purification. 2.2 Synthesis of polypyrrole nanotubes (PNTs) PNTs were prepared as the previous report with some modifications [5]. First, 0.35 g of MO and 2.8 g of FeCl3·6H2O were completely dissolved into 100 ml deionized water. Subsequently, 0.78 ml of pyrrole monomer was dropwise added into the mixed solution, and stirred for 24 h. The resulting products were washed with deionized water and ethanol for several times, and then dried at 60℃ for 10 h in a vacuum oven. 5

2.3 Synthesis of NiCo-MOF@PNTs Typically, 0.375 mmol CoCl2·6H2O and 0.375 mmol NiCl2·6H2O were dissolved into the mixed solution of N,N-dimethylformamide (DMF), ethanol and deionized water. 10 mg PNTs were then dispersed into the mixed solution by ultrasonic vibration for 30 min. Subsequently, 0.125 g p-phthalic acid (PTA) was added into the mixed solution and stirred for 1 h. 0.8 ml triethylamine (TEA) was quickly added, and the mixed solution was ultrasonicated for 8 h. The as-prepared products were washed with ethanol for 3 times, and dried in a vacuum oven at 100℃ for overnight, denoting as NiCoMOF@PNTs. For comparison, the pristine NiCo-MOF and NiCo-MOF with different account of PNTs were fabricated by the same experimental process. The detailed characterizations and electrochemical measurements can be referred to electrical supporting information.

3 Results and discussion 3.1 Morphology and structural characterization

Scheme 1 Scheme of the synthesis of NiCo-MOF@PNTs. The preparation process of NiCo-MOF@PNTs nanocomposites is illustrated in Scheme 1. First, Fe3+ and methyl orange formed the fibrillary reactive template, which directs the growth of PNTs and degrades automatically due to the reduction of oxidizing Fe3+ 6

cations [52]. Then, PNTs were dispersed in DMF solution containing Ni2+, Co2+, PTA, and TEA. The mixtures were ultrasonicated for 8 h to obtain ultrathin NiCo-MOF nanosheets wrapping PNTs.

Fig. 1 (a) SEM image, (b) and (c) TEM image, (d) HRTEM, and (e) the elementary mapping of NiCo-MOF@PNTs The microstructure and morphology of NiCo-MOF@PNTs are tested by SEM and TEM as shown in Fig. 1. Obviously, NiCo-MOF nanosheets are tightly and uniformly attached on the surface PNTs (Fig. 1a), which effectively hinders the agglomeration of NiCo-MOF nanosheets. Fig. S1 shows that the amounts of PNTs significantly affect the morphologies of NiCo-MOF@PNTs nanocomposites. With low PNTs contents, the NiCo-MOF@PNTs exhibits the similar morphology with the pristine NiCo-MOF, and PNTs distributed sporadically between NiCo-MOF nanosheets (Fig. S1 a-c). In contrast, with increasing PNTs contents, most of NiCo-MOF nanosheets were attached on the surface of PNTs (Fig. S1d-g). The elemental distribution of NiCo-MOF@PNTs was 7

investigated by energy dispersive X-ray spectroscopy (EDS). Fig. 1e shows the existence of N, Co, Ni, C and O elements that are uniformly distributed in the framework of NiCo-MOF@PNTs, and the atomic percentages of N, Co, Ni, C and O in NiCo-MOF@PNTs (Fig. S2) are 11.13%, 17.42%, 19.15%, 14.49% and 37.82%, respectively. The atomic ratio of Ni and Co is 1.099, which approximates to the molar ratio of Ni and Co salts. The TEM images of NiCo-MOF@PNTs are displayed in Fig. 1b and c. It further confirms that NiCo-MOF nanosheets were wrapped around PNTs with a diameter of about 200 nm, which matched well with SEM images. HRTEM (Fig. 1d) shows the existence of nanoclusters with the diameter of 3~5 nm on the edges of PNTs marked by red circles. The lattice fringes of MOF nanoclusters with the lattice spacing of 0.27 nm are associated with the (220) plane of Co-MOF [53].

Fig. 2 (a) X-ray diffraction patterns and (b) FT-IR spectrum of NiCo-MOF, NiCo8

MOF@PNTs and PNTs, (c) N2 sorption isotherms, and (d) pore size distribution of the NiCo-MOF, NiCo-MOF@PNTs and PNTs. The crystal structures of NiCo-MOF, NiCo-MOF@PNTs and PNTs were tested by XRD. As shown in Fig. 2a, PNTs exhibit a wide peak at 15˗30 º , which is the characteristic peak of PNTs [44]. NiCo-MOF and NiCo-MOF@PNTs exhibit the similar XRD patterns with NiCo-MOF reported in the previous report [54], indicating that PNTs have no obvious influence on the crystal structure of NiCo-MOFs. The FTIR spectra were performed to characterize the functional groups of the as-prepared samples. As shown in Fig. 2b, the pristine NiCo-MOF and NiCo-MOF@PNTs demonstrate similar patterns, suggesting the existence of NiCo-MOF in NiCoMOF@PNTs. The peaks at 3598 and 3430 cm-1 are assigned to O-H stretching vibration, resulting from the coordinated water in the MOFs [8, 55]. The strong peaks at 1577 and 1383 cm-1 are attributed to the asymmetric and symmetric stretching vibration of -COO group, respectively, indicating that –COO groups are coordinated to the metal center through bidentate mode [56]. The peaks at 815 and 753 cm-1 are assigned to the out-ofplane bending vibration of C-H [34, 57]. For the spectrum of PNTs, the peaks at 1541, 1457, 1295, and 1040 cm-1 are attributed to the C=C, C-C, C–H, and C–N stretching, respectively, while the peaks at 898 and 780 cm-1 correspond to the N–H (wagging) and N–H (out of plane) vibrations, respectively [58]. The specific surface area and pore structure were measured by N2 sorption isotherms as shown in Fig. 2c and d. The isotherm plots reveal the typical IV with the hysteresis loop at a relative pressure (P/P0) in the range of 0.7 ~ 1.0, indicating the mesoporous 9

structures [34]. The calculated specific surface areas are 33.1, 45.4, and 66.5 m2 g-1 for NiCo-MOF, NiCo-MOF@PNTs, and PNTs, respectively. The pore size distributions of all samples are centered at 2 ~ 5 nm, and the pore volume of NiCo-MOF@PNTs is larger than that of NiCo-MOF but smaller than that PNTs. Therefore, PNTs can improve the specific surface area as well as the electronic conductivity of NiCo-MOF nanosheets, thus enhancing the electrochemical performance.

Fig. 3 (a) XPS survey of the NiCo-MOF@PNTs, (b) Ni 2p, (c) Co 2p, (d) O 1s, (e) N 1s and (f) C 1s. The element composition and chemical state of NiCo-MOF@PNTs were examined by XPS as shown in Fig. 3. The survey spectrum of NiCo-MOF@PNTs shows the existence of Co, Ni, O, C and N elements (Fig. 3a), in which Ni, Co, and O elements originate from NiCo-MOF nanosheets, while N element originates from PNTs [59]. Fig. 3b shows that two strong peaks at 856.3 and 874.0 eV are attributed to the Ni 2p3/2 and Ni 2p1/2 of Ni2+ in NiCo-MOFs, and the other two peaks at 861.4 and 800.7 eV represent the satellite peaks of Ni 2p3/2 and Ni 2p1/2, respectively [60]. For the high resolution Co 10

2p spectrum in Fig. 3c, two strong peaks at 781.9 (Co 2p3/2) and 797.2 eV (Co 2p1/2), accompanying by two shake-up satellite peaks at 787.2 and 803.3 eV, indicate the presence of Co2+ in NiCo-MOF [61]. Fig. 3d shows three peaks centered at 533.8, 532.3, and 531.6 eV, corresponding to the hydroxyl, chemisorbed water, and oxygen coordination defects [19, 62], respectively. As shown in Fig. 3e, the N 1s spectrum consists of three main constituent peaks. The peaks at 400.0 and 402.0 eV correspond to the –NH– in the pyrrole ring, and the positively charged nitrogen (–NH+–), respectively, while the binding energies at 398.3 eV is attributed to the π* character bonding from –C=N–, which called pyridinic-N [43, 44]. Fig. 3f depicts two intense bands with binding energies of 284.6 and 288.9 eV are assigned to the aromatic linked carbon (C=C) and the carboxylate carbon (O–C=O), respectively [63]. 3.2 Electrochemical properties

Fig. 4 (a) CV curves at a scan rate of 20 mV s-1, and (b) GCD curves at a current density of 1 A g-1 for NiCo-MOF, NiCo-MOF@PNTs and PNTs; (c) CV curves of at various scan rates, and (d) GCD curves at various current densities of NiCo11

MOF@PNTs; (e) The specific capacitances at different current densities, and (f) Nyquist plots of NiCo-MOF, NiCo-MOF@PNTs and PNTs. The electrochemical performances of NiCo-MOF@PNTs, NiCo-MOF and PNTs were investigated by CV, GCD, and EIS in 2 M KOH. Fig. 4a shows CV curves of the three samples at a scan rate of 20 mV s-1 in the operating potential of 0~0.6 V (vs. Hg/HgO) without polarization reactions. The obvious redox peaks indicate the faradaic pseudocapacitance characteristics derived from the redox reactions of Ni2+/Ni3+ and Co2+/Co3+. Additionally, the NiCo-MOF@PNTs electrode exhibits the largest CV area, indicating the largest specific capacitance. The GCD curves were measured at 1 A g-1 in the potential range of 0~0.5 V. Clearly, the symmetric curves for all samples suggest the highly reversible redox reactions at the interface of electrode/electrolyte. Furthermore, Fig. 4b shows an obvious redox plateau in the range of 0.35 to 0.2 V, indicating the faradaic pseudocapacitance characteristics. In particular, NiCoMOF@PNTs exhibits the maximum discharge time, suggesting the superior capacitive performance. Fig. 4c shows the CV curves of NiCo-MOF@PNTs at various scan rates in the potential range of 0 to 0.6 V. It can be seen that NiCo-MOF@PNTs electrode still has an obvious redox peak and no significant polarization even at the scan rate of 50 mV s-1, implying the excellent rate capability of NiCo-MOF@PNTs [64]. The anodic and cathodic shifts are assigned to the rise of internal resistance with the scan rate [26]. Fig. 4d displays GCD curves of NiCo-MOF@PNTs at different current densities from 0.5 to 20 A g-1. The obvious redox plateaus can be reserved in the discharge curve even at high current density, implying the dominant pseudocapacitance 12

and outstanding rate capability [65]. The calculated specific capacitances from GCD curves were shown in Fig. 4e. NiCo-MOF@PNTs exhibited the maximum specific capacitance among all samples, and the specific capacitances were 1109, 998, 957 and 888 F g-1 at 0.5, 5, 10 and 20 A g-1, respectively. It reveals that the specific capacitance has no obvious reduction with the current density, demonstrating the excellent rate capability.

To

evaluate

the

ion

diffusion

and

electron

transfer

at

the

electrolyte/electrode interface, EIS measurements were carried out in the frequency range from 100 kHz to 0.01 Hz. Fig. 4f shows that the Nyquist plots consist of a semicircle and straight line. Obviously, the intercept on the real axis become shorter and the radius of the semicircle gradually decreases at the high-frequency region with PNTs contents, suggesting the low equivalent series resistance (Rs) and charge transfer resistance (Rct). According to the fitting circuit diagram presented in the inset of Fig. 4f, Rct values of PNTs, NiCo-MOF@PNTs and NiCo-MOF are 0.25, 0.41, and 0.64 Ω, respectively, and their Rs values are 0.51, 0.78, and 1.02 Ω, respectively. It indicates that PNTs can effectively enhance the electronic conductivity of NiCo-MOF. Additionally, the slopes of straight lines at the low-frequency region are similar, indicating that PNTs has slightly impact on the diffusion resistance. Besides, the cyclic stability of NiCo-MOF@PNTs in a three-electrode system is shown in Fig. S3. The capacitance retention rate was 81.4% after 5000 cycles, indicating a good cyclic stability. The crystal structure and morphology of NiCo-MOF@PNTs after cycling stability were further tested by XRD and SEM. The XRD patterns show that no impure phases are observed (Fig. S4), and it maintained the original morphology except the 13

slight aggregation (Fig. S5). The effect of PNTs contents on the capacitive performance of NiCo-MOF was investigated by the CV and GCD. Fig. S6 shows that all CV curves have similar shapes, and the oxidation and reduction peaks of all electrode materials are centered at about 0.5 and 0.2 V, respectively, demonstrating the similar pseudocapacitance behaviors. Additionally, the distance of oxidation and reduction peaks became shorter with the amount of PNTs at the same scan rate, demonstrating the higher electronic conductivity. The specific capacitances of NiCo-MOF with different PNTs contents were calculated from the GCD curves (Fig. S6), and the results are listed in Table S1. It shows that NiCo-MOF/PNTs exhibits the maximum specific capacitance with 10 mg PNTs, and then the specific capacitance decrease with increasing the PNTs contents due to the lower specific capacitance of PNTs compared with NiCo-MOF. As shown in Fig. S7a, the position of redox peak of PNTs has no obvious change with the scan rates, indicating that PNTs have high electronic conductivity and a good charge storage property with high rate performance [66]. Fig. S7b display the GCD curves of PNTs, and the calculated specific capacitance approximates 296 F g-1 at 1 A g-1, which provides the extra pseudocapacitance for electrode materials.

14

Fig. 5 (a) Schematic illustration of the NiCo-MOF@PNTs//AC ASC device, (b) CV curves of NiCo-MOF@PNTs and AC electrodes in a three-electrode system at a scan rate of 30 mV s-1, (c) CV curves of the ASC at a scan rate of 30 mV s-1 in the different 15

voltage windows, (d) CV curves of ASC at various scan rates, (e) GCD curves of ASC at various current densities, (f) the specific capacitance and colombic efficiency at different current densities, (g) Ragone plots of ASC, and (h) The cyclic stability at a current density of 5 A g-1 for 10000 cycles and the inset shows the last ten cycles. To evaluate the practical application of NiCo-MOF@PNTs in supercapacitors, Fig. 5a shows the assembled asymmetric supercapacitor (ASC) device using NiCoMOF@PNTs and activated carbon (AC) as the positive electrode and negative electrode, respectively, and the polypropylene diaphragm acts as a separator to prevent direct contact between positive and negative electrodes. The mass ratio of positive and negative electrodes depends on the number of charges stored in each electrode. The balance equation used is as follows: 𝒎 + 𝑪𝟑𝒆 + ∆𝑽 + = 𝒎 ― 𝑪𝟑𝒆 ― ∆𝑽 ― (1)

Where m+ and m- are the mass of active materials of the positive and negative electrodes, respectively. C3e+ and C3e- are the specific capacitance of the positive and negative electrodes, respectively. ΔV+ and ΔV- are the potential window of the positive and negative electrodes, respectively. According to the Eq. (1) and Fig. S8, the mass of NiCo-MOF@PNTs is 3 mg, and the corresponding mass of activated carbons should be 8.2 mg. Fig. 5b shows CV curves of NiCo-MOF@PNTs and AC at a scan rate of 30 mV s-1. The stable potential windows of AC and NiCo-MOF@PNTs electrode are in the range of -1.0~0.0 V and 0.0~0.6 V in a three-electrode system, respectively. Obviously, CV curves of positive and 16

negative electrodes exhibit the similar areas, and no polarization was observed. Fig. 5c shows CV curves of the ASC at different potential ranges. When the voltage window increases from 0 to 1.5 V, the shape of CV curve changes regularly, and there is no polarization phenomenon. Therefore, the electrochemical performance of ASC was tested at the potential window of 0~1.5 V. Fig. 5d displays CV curves of the ASC device in a voltage window of 0~1.5 V at the scan rates of 5 to 50 mV s-1. It shows that CV curves maintain a similar shape even at a scan rate of 50 mV s-1, demonstrating the excellent rate performance. The GCD curves of the ASC in the voltage window of 0~1.5 V are shown in Fig. 5e, and the calculated specific capacitance and columbic efficiency are demonstrated in Fig. 5f. The specific capacitance of the ASC is 132 F g-1 at 0.5 A g-1, and still retains 97.3 F g-1 at 10 A g-1, suggesting the outstanding rate capability. Additionally, the columbic efficiency remains almost unchanged even at the high current density, indicating excellent electrochemical reversibility of the ASC device. The Ragone plot is another significant factor to estimate the practical application of the ASC devices. As shown in Fig. 5g, the NiCo-MOF@PNTs//AC device achieves a high energy density of 41.2 Wh kg-1 at a power density of 375W kg-1, which is higher than the values of the previous report. Fig. 5h exhibits the cycling stability of the ASC device, and the inset is the last ten cycle of the whole charge-discharge curves. The capacitance retention of the NiCo-MOF@PNTs//AC device is 79.1% after the 10000 cycles, exhibiting the good cycling stability. In the last, two ASC devices were connected in series, which can light a yellow LED lamp for 30 min (Fig. S9). This is visual proof of the energy storage properties of our electrode materials. 17

4 Conclusions In brief, NiCo-MOF nanosheets wrapped PNTs hybrid were successfully synthesized via the facile ultrasonic method at room temperature. The elaborate structure of NiCoMOF@PNTs shows the excellent electrochemical performance. NiCo-MOF@PNTs with 10 mg of PNTs delivers the maximum specific capacitance of 1109 F g-1 at 0.5 A g-1, and superior rate capability. Furthermore, the assembled ASC device delivered the maximum energy density of 41.2 Wh kg-1 at a power density of 375W kg-1. The specific capacitance of ASC retains 79.1% of the initial capacitance over 10000 cycles, indicating the suitable cycling stability. The excellent capacitive performance of NiCoMOF@PNTs is attributed to the enhanced electronic conductivity and the more exposed active sites with PNTs. The results show that NiCo-MOF@PNTs is a promising candidate as electrode materials for high-performance supercapacitor.

Acknowledgment The work was financially supported by Natural Science Foundation of Shanxi Province (No. 201801D121284), and Program for the Outstanding Innovative Teams of Higher Learning Institutions of Shanxi.

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Graphical Abstract

NiCo-MOF nanosheets wrapping polypyrrole nanotubes for high-performance supercapacitors

A facile, cost-effective and green strategy was offered to efficiently enhance the electrical conductivity of NiCo-MOF nanosheets using emerging polypyrrole nanotubes with high conductivity by a facile ultrasonication. Compared with other conductive materials, PNTs can not only improve the active material's conductivity and prevent agglomeration, but also provide additional pseudocapacitance. The as-prepared ultrathin NiCo-MOF@PNTs composite exhibit a high specific capacitance of 1109 A g-1 at a current density of 0.5 A g-1. The assembled asymmetric supercapacitor (NiCo-MOF@PNTs//activated carbon) delivers an energy density of 41.2 Wh kg-1 at a power density of 375 W kg-1, and still holds 30.14 Wh kg-1 at 7.5 kW kg-1.

Highlights

NiCo-MOF nanosheets wrapping polypyrrole nanotubes for high-performance supercapacitors

· Ultrathin NiCo-MOF@PNTs composite were synthesized via an ultrasonic method. ·NiCo-MOF@PNTs electrodes have a high capacitance of 1109 F g-1 at 0.5 A g-1. · The supercapacitor show high energy density of 41.2 Wh kg-1 at 375 W kg-1. · The capacitance retention of supercapacitor device is 79.1 % after 10000 cycles.

Dear editor and referees: Here within enclosed is our paper for consideration to be published on "Applied Surface Science". No conflict of interest exits in the submission of this manuscript, and manuscript is approved by all authors for publication. The authors claim that none of the material in the paper has been published or is under consideration for publication elsewhere. If the article was accepted, it will not be published elsewhere in the same form, in English or in any other language, without the written consent of the Publisher.

Yours sincerely, Dr. Yanzhong Wang School of Materials Science and Engineering, North University of China, Taiyuan 030051, China *E-mail: [email protected] (Y. Z. Wang).

Authors contribution section

Yuexin Liu: Methodology, Writing- Original draft preparation Yanzhong Wang: Conceptualization, Supervision Yanjun Chen: Methodology, Formal analysis Chao Wang: Writing- Reviewing and Editing, Project administration Li Guo: Supervision, Funding acquisition