NiMoO4 nanowire arrays and carbon nanotubes film as advanced electrodes for high-performance supercapacitor

Accepted Manuscript Full Length Article NiMoO4 Nanowire Arrays and Carbon Nanotubes Film as Advanced Electrodes for High-performance Supercapacitor Si-Wen Zhang, Bo-Si Yin, Chang Liu, Zhen-Bo Wang, Da-Ming Gu PII: DOI: Reference:

S0169-4332(18)32008-7 https://doi.org/10.1016/j.apsusc.2018.07.110 APSUSC 39925

To appear in:

Applied Surface Science

Received Date: Revised Date: Accepted Date:

4 April 2018 21 June 2018 16 July 2018

Please cite this article as: S-W. Zhang, B-S. Yin, C. Liu, Z-B. Wang, D-M. Gu, NiMoO4 Nanowire Arrays and Carbon Nanotubes Film as Advanced Electrodes for High-performance Supercapacitor, Applied Surface Science (2018), doi: https://doi.org/10.1016/j.apsusc.2018.07.110

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NiMoO4 Nanowire Arrays and Carbon Nanotubes Film as Advanced Electrodes for High-performance Supercapacitor Si-Wen Zhang, Bo-Si Yin, Chang Liu, Zhen-Bo Wang*, Da-Ming Gu MIIT Key Laboratory of Critical Materials Technology for New Energy Conversion and Storage, School of Chemistry and Chemical Engineering, Harbin Institute of Technology, No.92 West-Da Zhi Street, Harbin, 150001 China * Corresponding author. Tel.: +86-451-86417853; Fax: +86-451-86418616. Email: [email protected] (Z.B. Wang)

ABSTRACT Self-supported NiMoO4 nanowire arrays (NNAs) are successfully grown on nickel foam (NF) via a simple one-step hydrothermal method and anneal treatment. Importantly, the in-situ growth method can reduce the contact resistance between the active material and current collector. It also can prevent the binder/conductive agent from “area of the deactivation” phenomenon. So, the design can effectively improve the electrochemical performance of electrode materials. In addition, a highly flexible carbon nanotubes (CNTs) film is also prepared by vacuum filtration step. Finally, an all-solid asymmetric supercapacitor (ASC) based on the optimized NiMoO4 nanowires electrode and carbon nanotube film (CNF) is assembled with PVA/KOH gel electrolyte. The ASC device presents outstanding electrochemical performance with a high energy density of 54.3 Wh kg-1 at a power density of 4344 W kg-1. The specific capacitance of the NNAs//CNF ASC remains 91.6% after 6000 cycles with a current density of 14 A g-1. At the same time, it also can continue to work under different bending states with almost constant performance. These results reveal that the NNAs//CNF ASC has great application prospect for flexible electronic fields.

KEYWORDS: nickel molybdate, CNTs film, supercapacitor, Ni foam 1

1. Introduction With ever-increasing demands of electric vehicles and portable electronic devices, new challenges have occurred on energy storage devices, especially the new-generation supercapacitors[1-11]. Supercapacitors also known as electrochemical capacitors have gained a substantial amount of research due to their rapid charge and discharge ability, high power density, and excellent cycle stability properties[12-18]. However, the low energy density has always been the main reason that limits the commercialization of supercapacitors. In order to improve the energy density of the supercapacitor, the selection of electrode materials plays an important role[19-21]. By now, transition-metal oxides/hydroxides, such as Ni(OH)2[22], Co3O4[23], and MnO2[24], are usually used as electrode materials because of their multiple oxidation states and the relatively low cost. However, their low electrical conductivity and poor structural instability will lead to a reduction of performance and limit their practical applications[25]. To avoid these limitations, many studies have focused on improving electrochemical performance of pseudocapacitors by concentrating on the exploitation of new electrode materials based on ternary metal oxides (NiCo2O4[26], NiMoO4[27], CoMoO4[28], ZnCo2O4[29],). Especially for Nickel molybdate (NiMoO4), as an emerging material, it has the advantages of reserve abundance, chemical stability and low price. More significantly, NiMoO4 has potential application value in future energy storage devices because it combines high theoretical capacitance from nickel atoms and excellent conductivity from molybdenum atoms. Guo et al. reported NiMoO4 nanowires supported by carbon cloth with a good cycling ability (specific capacitance remains 76.9% after 4000 cycles)[30]. Hong et al. showed that the 1-D HAR-NiMoO4 electrode exhibited an energy density of 40.7 Wh kg-1[31]. Huang et al. had synthesized 3D porous NiMoO4 nanoplate arrays with 87% retention of the initial specific capacitance after 3000 cycles. Although all of the above works are good, there is still room for improvement in both cycles performance and energy density. On the other hand, as we all 2

know, most of the supercapacitors often use activated carbon with binder/conductive agent as negative electrode. However, this approach will not allow sufficient contact between the electrolyte and the active materials duo to “area of the deactivation” phenomenon. Therefore, the design of a self-supported, high performance and flexible electrode is very necessary. In this work, a novel supercapacitor (SC) based on NiMoO4 nanowire arrays (NNAs) and carbon nanotube film (CNF) is assembled. The design has the following advantages: (1) the self-supported NNAs can easily permit the diffusion of the electrolyte into the electrode’s internal, thereby reducing the diffusion impedance of the electrolyte. (2) The NNAs on foam with strong adhesion can significantly shorten the distance of electron transport. (3) As a self-supported CNTs film, the electrode not only has the advantages as the above NNAs has, but also exhibits high flexibility. Based on the above advantages, our device has excellent performances. The NNAs//CNF asymmetric supercapacitor (ASC) with a maximum voltage of 1.45 V has exhibited a high energy density of 54.3 Wh kg-1 at a power density of 4344 W kg-1. With charging-discharging long-term cycle up to 6000 times, the ASC still remains 91.6% of its initial specific capacitance. It also processes a remarkable bend ability up to 120 o with no obvious structural damage. After fully charged with four devices in series, it can easily light up light-emitting diodes (LEDs). This strategy is expected to be a new favorite for flexible electronics in the future.

2.Experimental Synthesis of 1D NNAs on Ni Foam. First, tailor the Ni foam to 2h1h0.1 cm and clean them ultrasonically for 15 min in deionized (DI) water, ethanol, and acetone, respectively. Then, the substrate was placed against the wall of a Teflon-lined stainless steel autoclave that contained a homogeneous solution of Ni(NO3)2·6H2O (3 mmol) and Na2MoO4·2H2O (3 mmol) in a mixed solvent of 40 3

ml of DI water [32]. Afterwards, the autoclave was sealed and maintained at 150ϨC for 5 h to synthesize Ni-Mo precursor nanowire arrays. Then, the Ni foam was taken out, rinsed by ultrasonication in deionized water for 2 min and annealed at 300 oC in air for 2 h. The mass loading of the NiMoO4 nanowire arrays on the NF was calculated to be about 2.5 mg cm-2. Synthesis of self-supported CNF electrode The self-supported CNF electrode was prepared according to our previous experiment [4] In a typical synthesis procedure, CNF electrode was obtained by vacuum filtration (The CNTs were purchased by Time@nano). First, 1g dispersant and 60 mg CNT were dissolved in 100 mL deionized water under magnetically ultrasonic for 1 h in air. Finally, the as-obtained solution was filtered on the PTFE membrane filter and the film was then dried and could be easily peeled off from the filtration paper. Fabrication of NNAs//CNF supercapacitor The asymmetric supercapacitor (ASC) device was assembled by using NNAs as the positive electrode, CNF as the negative electrode, and cellulose paper as the separator. PVA/KOH gel as a solid electrolyte. PVA/KOH gel was prepared as follows: in a typical process, 4 g PVA was dissolved in 40 mL deionized water with stirring at 85 oC for 1 h. Then, 2 mol L-1 KOH was slowly dropped into the above solution at 80 oC under stirring until the solution became clear, respectively. The positive and negative electrodes were soaked in the gel for about 10 min and then were assembled together. The device was kept at 60 oC for 12 h to remove excess water in the electrolyte. The weight of device is 5.5mg.

3. Results and discussion NNAs are synthesized by a simple hydrothermal process with a calcination treatment. Fig. 1 shows a schematic diagram of the formation of NNAs growing on NF. In the first step, the mixture of solutions (Na2MoO4·2H2O and Ni(NO3)2·6H2O) can be formed with plenty of 4

small crystallites at room temperature. In the nucleation stage, the nucleation centers who serve for the growth of NiMoO4 nuclei is formed by self-assemble of Ni2+ and MoO42- ions. The original NiMoO4 crystals then form smaller nanowires in the hydrothermal conditions. As the reaction time extends, the nanowires grow longer by the nucleation mechanism[33]. The reaction equation is found as follows: Ni2+ + MoO42-

NiMoO4

The morphologies and structure in different reaction times are shown in Fig. 2. When the reaction time increases to 10 h, the skeleton of NF is partially covered and hardly seen (Fig. 2a-c). Further prolonging the reaction time to 15 h, the skeleton structure is completely invisible and some NiMoO4 nanoflowers are observed to overlap the nanowires (Fig. 2d-f). This shows that the structure gradually presents a flower-shape with the increasing reaction time. When the reaction time is 5 h, the microstructure and morphology of the NNAs on Ni foam are shown in Fig. 3a. However, it is obvious that the NNAs are evenly coated on the skeleton structure of foam nickel. The high-magnification SEM images (Fig. 3b,c) indicate that the average diameter of a single nanowire is about 100 nm. To prove the distribution of the elements, the energy dispersive spectroscopy (EDS) elemental mapping images of NNAs/NF are shown in Fig.3d-h, which confirm the uniform distribution of Ni, Mo, and O. The EDS spectra further illustrates the element composition of the nanowires in Fig. 3i. XPS spectra (Fig. 4) are demonstrated to estimate the detailed oxidation state and compositions of the as-prepared NNAs. From the full survey spectra of XPS (Fig. 4a), the signals of Mo, Ni, and O elements are clearly discovered in our sample, which match well with the results of EDS. The main binding energy peaks are clearly observed at Ni2p3/2 (864.2 eV) and at Ni2p1/2 (881.8 eV) with a Ni2+ oxidation state of 17.6 eV (Fig. 4b)[34]. In addition, two peaks of 238.7 and 241.7 eV are detected in the Mo 3d region (Fig. 4c) with the fission width of 3.0 eV revealing the Mo6+ oxidation state [35]. The O 1s spectrum (Fig. 4d) has just 5

one peak at 530.2 eV, which corresponds to the oxygen ions in oxides[36]. The phase and crystal structure of NNAs are found with XRD analysis in Fig. 5a. Expect for Ni peaks, all diffraction peaks are in good agreement with the monoclinic NiMoO4 (JCPDS, no. 86-0361)[37]. The Brunauer-Emmett-Teller (BET) surface area value is measured by the N2 adsorption-desorption isotherms (Fig. 5b, Fig S1a and Fig. S1c). NNAs (5h) have slightly larger BET surface area (9.43 m2g-1). The surface area of the as-prepared NNAs was much higher than the other samples (10h and 15h). At the same time, the Barrett-Joyner-Halenda (BJH) pore size distribution (Fig. 5c) indicates that the composite’s pore size is 2.3 nm. The porous structure can easily promote the progress of the electrolyte diffusion in/outward electrode materials, thus improves the electrochemical properties of the NNAs. The pore sizes of the samples (10h and 15h) are 2.5 and 2.7 nm in Fig.S1b,1d, respectively. The transmission electron microscopy (TEM) images for NNAs have been illustrated in Fig. 6. Fig. 6a depicts the low-magnification TEM image of NNAs. The size of the nanowire is about 100 nm (Fig. 6b), which is consistent with the previous SEM image results. The high-resolution TEM (HRTEM) image in Fig. 6c reveals the lattice fringes of 0.27 nm, corresponding to the (222) plane of NiMoO4. The selected-area electron diffraction (SAED) pattern (Fig. 6d) shows a well-defined lattice, indicating a single-crystal structure. The electrochemical properties of NNAs are measured by galvanostatic charge-discharge (GCD) and cyclic voltammetry (CV) in a three-electrode configuration using 2 mol KOH as the electrolyte. The GCD tests are executed within the voltage window of 0 to 0.45 V (vs.SCE) at current densities ranging from 15 to 35 mA cm-2. As shown in Fig. 7a, the GCD curves have two platforms with low IR drop and the curves are not linear, which show good pseudocapacitance characteristics. Fig. 7b shows the typical cyclic voltammetry (CV) curves of the NNAs electrode at different scan rates of 10, 30, 50 and 80 mV s-1. The shapes of the 6

CV curves further ascertain the pseudocapacitive characteristics. Specifically, a pair of redox peaks can be observed, which related to the Faradaic redox reactions of Ni (II) ↔Ni (III) + e-[38,39]. According to the results of CV curves, a potential energy storage mechanism is proposed in Fig. 7c. For NNAs electrode, it is well-accepted in alkaline electrolyte with the Faradaic reactions. At first, Nickel ions in nickel molybdate and hydroxyl ions can react to form nickel hydroxide, and then nickel hydroxide continues to react with the hydroxyl ions to produce nickel oxyhydroxide. The reaction equation is described as follows: Ni2+ + 2OH-

Ni(OH)2

Ni(OH)2+OH-

NiOOH+H2O+e-

As is known to all, electrochemical performance of electrode material mainly relies on its structure and size. The electrochemical tests (CV and GCD curves) of the NiMoO4-10h and NiMoO4-15h electrodes are shown in Fig. 8, respectively. For comparison, CV curves of NiMoO4-5h, NiMoO4-10h, NiMoO4-15h nanowires and pure NF at 10 mV s-1 are illustrated in Fig. 9a. The results reveal that the pure NF has little contribution to the total capacitance of the other samples. Besides, All the CV curves are similar with the redox peaks, but in the closed area, it is obvious that the NiMoO4-5h electrode is larger than the others, which reflects the best electrochemical behavior. The contrast of this nonlinear GCD curves is shown in Fig. 9b. Similarly, these curve platforms are corresponding to the CV curves. And it can be seen that the discharging time of NiMoO4-5h is the longer than other two materials. The corresponding areal capacitance values (Fig. 9c) of three samples are calculated by Eqn. (2)(in supporting information). It is obvious that the NiMoO4-5h electrode presents a higher areal capacitance (2.9 F cm-2) than the NiMoO4-10h and NiMoO4-15h electrodes. In addition, the NiMoO4-5h electrode still exhibits high specific capacitances (Fig. S2) of 1173, 1146, 1044, 773 and 591 F g-1 at discharge current densities of 6, 8, 10, 12 and 14 A g-1, respectively. The conductivity of the electrode material is also an important factor for the electrochemical 7

performance, and the electrochemical impedance spectra of the samples are tested in Fig. 9d. The NiMoO4-5h electrode demonstrates a smaller internal resistance (0.52 Ω) compared with those of NiMoO4-10h (0.56 Ω) and NiMoO4-15h (0.64 Ω). But above all, the NiMoO4-5h nanowires can be used as the best electrode material in this case. To further evaluate the practical applications of NNAs electrode, an asymmetric supercapacitor device is constructed by using the NNAs electrode as the positive electrode and self-supported CNF as the negative electrode. Before testing, to achieve the highest range of the potential window, the mass ratio of the positive electrode and the negative electrode was determined by equations (5)(in supporting information). The optimum loading ratio of NNAs to CNF is 1:4.48. On that basis, the flexible test of the device is shown in Fig. 10. The CV and GCD curves (Fig. 10 f,g) are not affected by the different bending angles (Fig. 10 a-e), indicating that the structural integrity of the device is not destroyed after bending. For the CNF electrode, we also find that it has high flexibility (Fig. 10 h,i). This will make a good match with NNAs electrode to form a flexible device. The electrochemical properties of CNF electrode were also examined by CV and GCD curves (Fig. S3a, S3b). There are the rectangular CV and linear GCD curves from -1.0 to 0 V for the CNF electrode, which is indicative of the EDLC behavior. According to the formula (1) (in supporting information), the specific capacitances (Fig. S3c) of the CNF electrode are calculated to be 101, 98, 92, 77 and 60 F g-1 at current densities of 1, 2, 3.5, 4.5 and 6 A g-1, respectively. The structure of the flexible device is shown in Fig. 11a. The potential windows (Fig. 11b) of the CNF and NNAs electrodes are -1.0 to 0 and 0 to 0.45 V, respectively. Therefore, the asymmetric supercapacitor can work at a voltage of 1.45 V. The background of the Fig. 11b is a digital photo of the solid-state supercapacitor. The CV and GCD curves of the NNAs//CNF ASC are revealed at different voltage windows from 1.0 to 1.8 V in Fig. 11c, d. As expected, when the voltage reached 1.8V, the shape of the CV and GCD curves does not change, which reveals 8

good electrochemical stability. The symmetric GCD curves of device (Fig. 11e) show the characteristic of fast charge and discharge rate. Fig. 11f displays the CV curves of the NNAs//CNF ASC device at various scan rates. All the CV curves exhibit quasi-rectangular shapes that represents typical capacitive characteristics. The device also has a low impedance as shown in Fig. 11g. Based on the GCD curves, as shown in Fig. 11h, we calculate its specific capacitance which can achieved 186, 154, 96, 50, 30 F g-1. As a device, cyclic performance is also critical. Fig. 11i exhibits the cycling performance of NNAs//CNF ASC measured at a current density of 14 A g-1 up to 6000 cycles. It can be noticed that the capacitance retains 91.6% of its initial capacitance, which suggests the excellent electrochemical stability of the ASC device. After long-term cycling, the structure of NNAs is maintained with little structural deformation caused by too many times of redox reaction (Fig. S4), which demonstrates that NNAs have better durability. According to Eqn. (3) and (4), the specific power density and energy density of the NNAs//CNF ASC are measured. The ASC displays a high energy density of 54.3 Wh kg-1 at a power density of 4344 W kg-1. As shown in the Ragone plot of Fig. 12a, compared with other previous reports about asymmetric supercapacitor devices, like NiMoO4-rGO//N-doped graphene (NG) (30.3 Wh kg-1 at 187 W kg-1) [40], CoMoO4–3D graphene//AC (21.1 Wh kg-1 at

300

W

kg-1)

[41],

NiMoO4//AC

(20.1

Wh

kg-1 at

2100

W

kg-1)[42],

MnFe2O4/graphene//AC(5 Wh kg-1 at 400 W kg-1)[43], Graphene/MnO2//AC(6.8 Wh kg-1 at 62 W kg-1)[44], VGO paper//AC (12.5 Wh kg-1 at 13.3 W kg-1)[45]. Additionally, we have demonstrated that our optimized four ASCs connected in series can successfully power up 40 red light-emitting diodes (LEDs) and a glow light (inset of Fig.12a and Fig. 12b). It can easily drive and keep the brightness even at bended state as observed in Fig. S5.

9

4. Conclusions In summary, the nanostructured NNAs/NF composite electrodes are constructed by a facile one-step method. The electrochemical performance can be optimized by the reaction time, and the sample shows a highest areal capacitance of 2.9 F cm-2 with the reaction time of 5

hours.

In

addition,

the

highly

flexible

CMF

is

also

synthesized

by

a

vacuum filtration method. Finally, these two components are assembled to form a solid-state supercapacitor with high electrochemical properties. The NNAs//CNF ASC manifests an excellent specific capacitance as high as 186 F g-1 at 6 A g-1 and a high energy density of 54.3 Wh kg-1 at a power density of 4344 W kg-1. Therefore, we believe the device is expected to be used in new-style flexible electronic products.

Acknowledgements We acknowledge the National Natural Science Foundation of China (Grant No. 21273058 and 21673064), China postdoctoral science foundation (Grant No.2012M520731), and HIT Environment and Ecology Innovation Special Funds (Grant No. HSCJ201620) for their financial support.

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(38) D. Ghosh, S. Giri, C.K. Das, Synthesis: characterization and electrochemical performance of graphene decorated with 1D NiMoO4-nH2O nanorods. Nanoscale 5 (2013) 10428-10437. (39) D. Cai, D. Wang, B. Liu, Y. Wang, Y. Liu, L. Wang, H. Li, H. Huang, Q. Li, T. Wang, Comparison of the Electrochemical Performance of NiMoO4 Nanorods and Hierarchical Nanospheres for Supercapacitor Applications. ACS Appl. Mater. Interfaces 5 (2013) 12905-12910. (40) T. Liu, H. Chai, D.Z. Jia, Y. Su, T. Wang, W.Y. Zhou, Rapid synthesis of mesoporous NiMoO4 nanorod/ reduced

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high-performance supercapacitors. Electrochim. Acta. 180 (2015) 998-1006. (41) X.Z. Yu, B.A. Lu, Z. Xu, Super Long-Life Supercapacitors Based on the Construction of Nanohoneycomb-Like Strongly Coupled CoMoO4-3D Graphene Hybrid Electrodes. Adv. Mater. 26 (2014) 1044-1051. (42) D.P. Cai, D.D. Wang, B. Liu, Y.R. Wang, Y. Liu, L.L. Wang, H. Li, H. Huang, Q.H. Li, T.H. Wang, Comparison of the Electrochemical Performance of NiMoO4 Nanorods and Hierarchical Nanospheres for Supercapacitor Applications. ACS Appl. Mater. Interfaces 5 (2013) 12905䳼12910. (43) W. Cai, T. Lai, W. Dai, J. A. Ye, facile approach to fabricate flexible all-solid-state supercapacitors based on MnFe2O4/graphene hybrids. J. Power Sources 255 (2014) 170-178. (44) Y. He, W. Chen, X. Li, Z. Zhang, J. Fu, C. Zhao, E. Xie, Freestanding Three-Dimensional Graphene/MnO2 Composite Networks As Ultralight and Flexible Supercapacitor Electrodes. ACS Nano, 7 (2013) 174-182. (45) C.Y. Foo, A. Sumboja, D.J.H. Tan, J. Wang, P.S. Lee, Flexible and Highly Scalable V2O5-rGO Electrodes in an Organic Electrolyte for Supercapacitor Devices. Adv. Energy 15

Mater. 4 (2014) 1400236.

16

Ni(NO3)2▪6H2O

MoO42-

Ni2+ MoO

Ni Foam

24

Na2MoO4▪2H2O

NiMoO4 nanowires

Hydrothermal

Calcination

Fig. 1 Schematic depicting of the fabrication procedure of the NNAs.

17

(a)

(b)

(c)

10μm

20μm

(f)

(e)

(d)

5μm

10μm

20μm

Fig. 2 SEM images of NiMoO4 nanowires on NFs obtained at (a-c) 10 h, (d-f) 15 h.

18

5μm

(b)

(a)

(c)

1μm

20μm

(d)

(e)

(g)

(h)

200nm

(f)

(i)

Fig. 3 SEM images of NNAs obtained in 5 h (a,b) Low-magnification (c) High-magnification. (d-h) SEM images and EDS elemental mapping images of Ni, Mo, and O. (i) EDS spectra of NNAs.

19

(a) O1s

Ni2p Ni2p

800

600

C1s

Mo3d

Intensity(a.u.)

Ni 2p1/2 Mo3p Mo3p

Intensity(a.u.)

1000

Ni 2p3/2

(b)

400

200

0

900

890

870

860

Binding energy (eV)

Binding energy (eV)

(c)

880

O 1s

(d)

Mo 3d5/2

Intensity(a.u.)

Intensity(a.u.)

Mo 3d3/2

250 248 246 244 242 240 238 236 234 232 230

540

535

530

525

Binding energy (eV)

Binding energy (eV)

Fig. 4 (a) XPS survey spectrum for NNAs. (b) Ni 2p, (c) Mo 3d, and (d) O 1s regions

20

850

Ni

Ni

10

20

30

40

50

(530)

(-424) (-532)

(330)

(-222) (-132)

(-201)

(-112) (220)

Intensity(a.u.)

(a)

60

70

2θ(degree)

(c)

(b) 0.004

15

Volume Adsorbed (cm3g-1)

Volume Adsorbed (cm3g-1)

18

12 9 6 3 0 0.0

0.2

0.4

0.6

0.8

0.002

0.000

0

1.0

20

40

Pore Size(nm)

-1 0

Relative Pressure (P·P )

Fig. 5 (a) XRD pattern of the NNAs. (b) Nitrogen adsorption-desorption isotherms of the NNAs (c) BJH pore size distributions plots.

21

(a)

(b)

50nm

200nm

(c)

0.27nm

(d)

5nm

5 1/nm

Fig. 6 (a,b) TEM images of NNAs. (c) HRTEM image of NNAs. (d) SAED pattern

22

0.5

0.3 0.2 0.1

(b)

10mV s-1 30 mV s-1 50 mV s-1 80 mV s-1

0.20 0.15 Current(A)

Potential(V)

0.4

0.0

0.25

15 mA cm-2 20 mA cm-2 25 mA cm-2 30 mA cm-2 35 mA cm-2

(a)

0.10 0.05 0.00 -0.05 -0.10 -0.15

0

20

40

60

0.0

80 100 120 140 160 180 Time (s)

0.1

0.2

0.3

0.4

0.5

Potential(V)

(c)

Fig. 7 (a) CV curves of the NNAs electrode at various scan rates. (b) Charge-discharge curves of the NNAs electrode at different current densities.(c) Schematic illustration for the electrochemical reaction process of the NNAs electrode.

23

0.5

0.10 10mV s-1 30 mV s-1 50 mV s-1 80 mV s-1

0.08

15 mA cm-2 20 mA cm-2 25 mA cm-2 30 mA cm-2 35 mA cm-2

0.4

0.04

Potential(V)

Current(A)

0.06

(b)

(a)

0.02 0.00 -0.02

0.3 0.2 0.1

-0.04 -0.06

0.0

0.0

0.1

0.2

0.3

0.4

0.5

0

50

Potential(V)

0.5

0.15

15 mA cm-2 20 mA cm-2 25 mA cm-2 30 mA cm-2 35 mA cm-2

0.4 Potential(V)

Current(A)

0.05

(d)

(c)

10mV s-1 30 mV s-1 50 mV s-1 80 mV s-1

0.10

100

Time (s)

0.00 -0.05 -0.10

0.3 0.2 0.1

-0.15 0.0 0.0

0.1

0.2

0.3

0.4

0.5

0

20

40

60

80

100

120

Time (s)

Potential(V)

Fig. 8 (a,b) CV and GCD curves of the NiMoO4@NF-10h electrode. (c,d) CV and GCD curves of the NiMoO4@NF-15h electrode.

24

0.08 0.06 0.04

0.5

(a)

15h 10h 5h Ni

0.02 0.00

5 h 10 h 15 h

0.3 0.2

-0.02 -0.04

0.1

-0.06 -0.08

0.0 0.0

(b)

0.4

Potential(V)

Current(A)

0.12 0.10

0.1

0.2

0.3

0.4

0

0.5

20

40

60

80 100 120 140 160 180 Time (s)

Potential(V)

(c)

2.5

(d)

6

15h 10h 5h

5h 10h 15h

5 2.0

4

1.5

1.8

5h 10h 15h

1.6 1.4

3

1.2 -Z''(Ω)

2.0

-Z''(Ω)

Areal capacitance(F cm-2)

3.0

2

1.0 0.8 0.6

1.0

0.4 0.2

1

0.0 0.5 0.6 0.7 0.8 0.9 1.0 1.1 1.2 1.3 1.4 1.5

0.5

Z'(Ω)

15

20

25

30

0

35

0

1

2

3

4

5

6

Z'(Ω)

Current density(mA cm-2)

Fig. 9 (a) CV curves of the NNAs electrodes prepared at different reaction time at the same scan rate of 10 mV s-1. (b) GCD curves of three samples. (c) Areal capacitances at different current densities of three samples. (d) Electrochemical impedance spectra of three samples .

25

(a)

0o 30o 60o 90o 120o

1.4 (f) 1.2 1.0 0.8 0.6 0.4

0.015 0.010 0.005

0o 30o 60o 90o 120o

(g)

0.000 -0.005 -0.010

0.2

-0.015

0.0 -0.2

0.020

Current(A)

1.6

Potential(V)

(c)

(b)

0

5

10

15

20

25

30

-0.020 -0.2 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 Potential(V)

Time (s)

(e)

(d)

(h)

(i)

(j)

Fig. 10 (a-e) The digital photographs of ASC in different bending angles. (f,g) GCD and CV curves of the ASC under different bending angles.(h-j) Digital photos of flexible CMF electrode.

26

(a)

0.025

2.0

0.015

NiMoO4 nanowires

CNTs Film

0.005 0.000 -0.005

0-1.0V 0-1.2V 0-1.4V 0-1.6V 0-1.8V

-0.010

current collector

-0.015 -0.020 0.0

1.0

0.5

0.0

2.0

0

5

10

0.0

0.2

0.4

1.0 0.8 0.6

0.010

0.000 -0.005

0.2

-0.015 0

50

100

(f)

0.005

-0.010

0.0

-0.020 -0.2 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 Potential(V)

Time (s)

(g)

(i)

100

0

20

40

60

80

Z'(Ω)

200

(h)

175 150 125 100

80 60 40

1.6

1.6

1.4

1.4

1.2

1.2

1.0

1.0 Potential(V)

10

Potential(V)

20

Capacitance retention(%)

30

0.8 0.6 0.4 0.2

20

20

5 mV s-1 10mV s-1 30mV s-1 50mV s-1 80mV s-1

0.015

0.4

40

0

0.020

(e)

6A g-1 8A g-1 10A g-1 12A g-1 14A g-1

1.4 1.2

-1.0 -0.8 -0.6 -0.4 -0.2

15

Time (s)

Current(A)

CNTs NiMoO4/Ni

Potential(V)

-Z''(Ω)

1.5

1.6

0.12 0.10 0.08 0.06 0.04 0.02 0.00 -0.02 -0.04 -0.06 -0.08

Potential(V)

Current(A)

0.5

1.0

Potential(V)

(b)

Specific capacitance(F g-1)

0-1.0V 0-1.2V 0-1.4V 0-1.6V 0-1.8V

1.5

0.010 Current(A)

separator

(d)

(c)

0.020

Potential(V)

current collector

0.8 0.6 0.4 0.2

0.0

0.0

-0.2

-0.2

0

5

10

15

20

25

30

35

40

0

5

Time (s)

10

15

20

25

30

35

Time (s)

75

0

50 25

0

1000

2000

0 6

8

10

12

3000

4000

5000

6000

Cycle number

14

Current density(A g-1)

Fig. 11 (a) Schematic illustration of the fabrication process of the NNAs//CNF ASC. (b) CV curves of the NiMoO4/Ni and CMF electrodes in a three-electrode system. (c,d) CV and GCD curves of the NNAs//CNF ASC at different voltage windows. (e,f) CV and GCD curves of the NNAs//CNF ASC. (g) Nyquist plots of ASC. (h) Specific capacitance of the ASC. (i) Cycling stability of the ASC 27

103

(b)

Energy density(Wh kg-1)

(a) This work

102 Ref.45

Ref.40 Ref.41

101 100 10-1 10-2 100

Ref.42 This work Ref.44 Ref.40 Ref.41 Ref.42 Ref.43 Ref.44 Ref.45

101

Ref.43

102 103 104 Power density(W kg-1)

105

Fig. 12 (a) Ragone plots of NNAs//CNF ASC and other ASCs reported previously for comparison. (b) The glow lights powered by four NNAs//CNF ASC in series.

28

Graphical abstract

29

Highlights g

High-performance All Solid-state Asymmetric Supercapacitor is designed.

g

The ASC presents outstanding electrochemical performance with a high energy density of

54.3 Wh kg-1

g

The specific capacitance of the NNAs//CNF ASC remains 91.6% after 6000 cycles

30