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Page 1 of 26

Dalton Transactions View Article Online

DOI: 10.1039/C8DT03598A

Wearable super-high specific performance supercapacitors using a honeycomb with folded silk-like composite by NiCo2O4 nanoplates

Published on 05 October 2018. Downloaded on 10/15/2018 2:20:01 AM.

Yedluri Anil Kumar* and Hee-Je Kim a

School of Electrical Engineering, Pusan National University, Busandaehak-ro 63beon-gil, Geumjeong-gu, Busan, 46241, Rep. of KOREA *

Corresponding Author. Tel: +82 51 510 2364. Fax: +82 51 513 0212.

E-mail: [email protected] (H.-J. Kim); [email protected] (A. K. Yedluri)

Dalton Transactions Accepted Manuscript

decorated with NiMoO4 honeycombs on nickel foam

Dalton Transactions

Page 2 of 26 View Article Online

DOI: 10.1039/C8DT03598A

ABSTRACT A novel binder-free electrode material and multicomponent design of NiCo2O4 (NCO) nanoplates adhered to NiMoO4 (NMO) honeycomb composites was prepared on nickel foam

honeycomb with folded silk-like NF@NMO@NCO nanostructures on nickel foam and its use to


increase the availability of electrochemically active sites to provide more pathways for electron transport and improve the utilization rate of the electrode materials. As a result, the as-fabricated NF@NMO@NCO electrode exhibited a maximum specific capacitance of 2695 F g-1 at a current density of 20 mA g-2, which is much better than that of NF@NCO nanoplates (1018 F g-1) and NF@NMO honeycomb (1194 F g-1). Moreover, the as-synthesized NF@NMO@NCO achieved a high energy density of 61.2 W h kg-1 and outstanding power density of 371.5 W kg-1 as well as exceptional capacitance retention of 98.9% after 3000 cycles. The outstanding electrochemical performance makes the honeycomb with a folded silk-like nanostructure a promising candidate for advanced electrochemical energy storage.

Keywords: Honeycomb with folded silk-like structure, honeycomb structure, nanoplate’s structure, Supercapacitors and Electrochemical performances.


(NF) using a simple chemical bath deposition strategy. This paper reports the synthesis of a

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DOI: 10.1039/C8DT03598A

Introduction The increasing energy consumption needs and fossil fuel crises has driven the development of

clean manner.1 Supercapacitors (SCs) or electrochemical capacitors have attracted considerable


interest as a power source owing to their remarkable characteristics (e.g. long cycling life time, high charge-discharge rate, and high power density), safer operation, lower maintenance cost, and high rate capability.2,3 The technology of energy storage devices is of upmost importance to fulfill the needs for portable and wearable electronic products, sensing devices, electric vehicles, and smart textiles.4,5 To design wearable supercapacitors, several conducting-based substrates, such as Ni foam, carbon cloth, graphene-coated fabric, carbon fibers, and Ti foil have been utilized as current collectors for flexible supercapacitor applications.6 Among them, Ni foam, with its high flexibility, high surface area, and extraordinary mechanical performance, can be used as a suitable conductive substrate for the growth of inexpensive electrode materials for supercapacitor applications. The electrode material is a key application that has an impact on the abilities of supercapacitors. According to the charge storage mechanism, supercapacitors can be divided mainly into two categories, such as electric double layer capacitors (EDLCs) and faradaic pseudocapacitors (PCs).7 EDLCs include mainly carbon-based materials, such as carbon nanotubes (CNTs), carbon spheres, activated carbon, reduced graphene, and porous carbon.8,9 The electric double layer capacitance gives rise to a low specific capacitance, low energy density, and volumetric capacitances, which have hindered their further practical use in supercapacitors.10 In contrast, PCs outperform EDLCs because of their reversible faradaic redox reactions and extraordinary discharge capacity.11 Moreover, they have higher energy storage capacity, higher


renewable energy sources and next generation energy storage devices in a more efficient and

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electrochemical performance, and multiple oxidation states for redox-reaction-enriched energy storage.12 Recently, transition metal oxides have been studied extensively for pseudo-capacitive

materials, e.g., NiO, RuO2, NiCoS4, MnO2 and Co3O4, have been considered the most attractive candidates.14,15 They have a much high specific capacitance, good rate capability and are


promising electrode materials with low cost. Until now, binary metal oxides have attracted considerable attention from the scientific community. Their high electrical conductivity and remarkable specific capacity are higher than those of single component oxides because of their achievable oxidation states multiple redox reaction.16 Moreover, binary metal oxides have many advantages, such as environmental friendliness, low cost, and abundant resources.17 Recently published articles have shown that binary metal oxides, such as nickel cobaltite (NiCo2O4), ZnCo2O4, Zn2SnO4, CoMoO4, NiMnO3, and NiMoO4, are promising materials that exhibit better electrochemical performance and are scalable alternatives because of their achievable oxidation states, high electrical conductivity, ample surface active sites, and strong permeability.18,19 Therefore, considerable effort has been made to fabricate different bimetallic oxide nanomaterials for supercapacitor applications with good capability. Among the binary metal oxides, NiCo2O4 have attracted considerable attention as one of the most promising and scalable electrode materials for supercapacitor applications owing to its good electrical activity, environmental friendliness, and excellent electrical conductivity.20 Zhang et al. selective synthesized hierarchical mesoporous spinal NiCo2O4 for high-performance supercapacitor applications, and reported a high capacitance of 1619.1 F g-1 at 2.0 A g-1.21 Yan et al. produced NiCo2O4 containing oxygen vacancies as a higher performance electrode material


or battery-type faradaic electrode applications.13 In the past few years, various faradaic electrode

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for supercapacitor applications by an electro-deposition method combined with a thermal treatment, showing a capacitance of 1590 F g-1 at a current density of 1 A g-1.22 Venkatachalam

supercapacitor electrode using a hydrothermal method, which delivered a capacitance of 767.5 F g-1 at a current density of 0.5 A g-1 for supercapacitor applications.23 Uke et al. reported the cost-


effective synthesis of spinel NiCo2O4 nanocrystals using a sol-gel citrate method and its supercapacitor applications and reported a high capacitance of 342 F g-1 at 1 mVs-1.24 Huang et al. reported facile synthesized NiCo2O4 nanoparticles as an electrode material for supercapacitor applications and obtained a high specific capacitance of 429.6 F g-1 at a current density of 1 A g1 25

.

Recently, Zhao et al. produced NiCo2O4 nanowire-based flexible electrode materials for

asymmetric supercapacitor applications, and reported a maximum specific capacitance of 830.8 F g-1 at 2 A g-1.26 Despite this progress, in most cases, the performance of NiCo2O4 is still unsatisfactory, which has restricted the performance of supercapacitors, particularly at chargedischarge curves of the specific capacitance process.27 To further increase the electrochemical performance, two types of binary metal oxides have been combined to enhance the supercapacitor performance.28 For example, Gu et al. reported NiCo2O4@MnMoO4 core-shell flowers for high performance supercapacitor applications by a hydrothermal method, showing a capacitance of 1118 F g-1 at a current density of 1 A g-1.29 In another study, Zhang et al. reported the facile synthesis of NiMoO4@CoMoO4 hierarchical nanospheres on Ni foam with a high specific capacitance of 1601.6 F g-1 at a current density of 2 A g-1.30 A rational structure design of electrode is also important for its electrochemical performance. A Ni foam substrate is normally used as a suitable conducting substrate owing its high conductivity and flexibility and lower fabrication cost because no binders are necessary.31-33


et al. synthesized a hexagonal-like NiCo2O4 nanostructure-based high performance

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Despite this progress, the performance of metal oxide-based electrodes are still unsatisfactory in most cases.34 On the other hand, it is still a challenge to produce metal oxides with various

should also be an excellent electroactive composite material. The combination with other


materials can enhance the supercapacitors performance further owing to the synergistic effect. In this study, NF@NiMoO4@NiCo2O4 honeycombs folded with silk-like structures was fabricated on Ni foam as a binder-free electrode using a simple chemical bath deposition process. The unique and stable morphology of the NF@NMO@NCO honeycomb structures allowed multiple large contact areas and a high-efficiency electron pathways that facilitates ion and electron transfer for the electrochemical reactions. The as-fabricated Ni foam-supported NF@NMO@NCO nanostructure achieved a very high specific capacitance of 2695 F g-1 at 20 mA g-2, which was higher than those of NF@NCO (1018 F g-1) and NF@NMO (1194 F g-1) nanostructures, as well as long cycling stability (98.9% after 3000 cycles). Moreover, the supercapacitor showed a high energy density of 61.2 W h kg-1 at a power density of 371.5 W kg1

. The electrode material with NF@NMO@NCO honeycomb folded with silk-like

nanostructures not only provided a large surface area, but also has a synergistic effect from NF@NCO and NF@NMO on the energy storage behavior as electrode materials for pseudocapacitor applications.

Experimental Section Material preparation.


configurations to enhance the supercapacitor performance. The similar NiMoO4@NiCo2O4

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All chemicals used in this study were of analytical grade and used as received. NF@NiMoO4@NiCo2O4 nanostructures were synthesized using a simple and cost effective

Synthesis of the NF@NiMoO4


First, a uniform NF@NiMoO4 electrode was synthesized using a simple chemical bath deposition (CBD) process combined with a subsequent annealing treatment and grown on nickel foam to form NF@NiMoO4 nanostructure. In a typical procedure, a piece of nickel foam used as a conductive substrate (1.5 cm × 4.0 cm in a rectangular shape) was etched ultrasonically with a 3M HCl solution for 30 min to remove the NiO layer from the surface, and then cleaned with deionized water and ethanol .Subsequently, the Ni foam was dried with a hair dryer for 20 min. In a typical experiment, the reaction solution was obtained by mixing 0.03 M of Na2MoO4.2H2O and 0.015 M of Ni(NO3)2.6H2O were dissolved in 50 mL of distilled water with stirring for 30 min. Subsequently, 0.3 M of urea (CH4N2O) and 0.06 M of ammonium fluoride (NH4F) were added to the mixed solution for 30 min at room temperature with vigorous stirring. The solution was then transferred to a 50 mL of capped bottle into which the cleaned Ni foams were immersed vertically. The bottle was then capped and maintained at 100 ℃ for 13 h in an electric oven. After the reaction for 13 h, the electrodes were removed from the electric oven, cooled naturally to room temperature and then washed several times with deionized water and ethanol. The resulting Ni foams were dried at 100 ℃ for seven hours in an electric oven. The collected NF@NiCo2O4 foams were annealed at 400 ℃ for 2 h to obtain the final material. The active electrode material NF@NiCo2O4 was synthesized using the same procedure for the synthesis of NF@NiMoO4 except for the addition of 0.03 M of Co(NO2)3.6H2O instead of 0.03 M of Na2MoO4.2H2O. Synthesis of the NF@NiMoO4@NiCo2O4 composite material.


chemical bath deposition method without structure-directing agents.

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In a typical synthesis, 0.03 M of Na2MoO4.2H2O, 0.015 M of Ni (NO3)2.6H2O, 0.3 M of urea (CH4N2O) and 0.06 M of ammonium fluoride (NH4F) were dissolved into 50 mL of deionized

the reaction solution. The homogeneous solution was then transferred to a 100 mL bottle and kept at 100 ℃ for 5 h. After being cool down to room temperature naturally, the as-prepared


samples was rinsed several times with deionized water and ethanol and dried at 60 ℃ for 12 h. The mass loading of the NiMoO4 and NiCo2O4 nanostructures on NF were around 9.0 and 7.0 mg cm-2.

Material Characterizations. The crystal structures and phase of the materials were examined by X-ray diffraction (XRD, D8 ADVANCE, Bruker) using a Cu Kα radiation at 40 KV and 40 mA. Field emission scanning electron microscopy (FE-SEM, S-4800, Hitachi) was used to examine the morphology and microstructures of the as-prepared materials. Transmission electron microscopy (TEM) images were taken on a JEOL, JEM-2100F microscope. The chemical compositions and chemical valences states were determined by X-ray photoelectron spectroscopy (XPS VG Scientific ESCALAB 250).

Electrochemical measurements. Electrochemical measurements were carried using a BioLogic-SP 150 workstation with a typical three-electrode system in a 3 M KOH aqueous electrolyte at ambient temperature. A threeelectrode configuration was used for cyclic voltammetry (CV) and galvanic charge-discharge (GCD) on an electrochemical workstation. Electrochemical impedance spectroscopy (EIS) was also carried out over the frequency range, 100 KHz to 0.1 Hz, with an amplitude of 5 mV.


water. Furthermore, a piece of the NiCo2O4 electrode-supported nickel foam was immersed in

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During the test, a piece of NF@NiMoO4@NiCo2O4 sample was used directly as the working electrode or active electrode, platinum foam (Pt) wire, and Ag/AgCl electrodes as a counter

materials was calculated from the galvanostatic charge-discharge curves, energy density, E, and


power density, P, using the following equations:

Csc = 2 ×

E=

×

. × × ∆ .

P = × ∆

(1)

×

(2)

(3)

where I (A) is the discharge current applied to the electrode, ∆v (V) is the potential drop, m (g) is the mass of the electrode material, and ∆ (S) is the discharge time from high to low potential, respectively.35

Results and Discussion


electrode and reference electrode, respectively. The specific capacitance of the electrode

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Scheme. 1 Schematic representation of the synthesis of honeycomb with folded silk-like NF@NiMoO4@NiCo2O4 nanostructure, honeycomb-like NF@NiMoO4 structure and nanoplatelike NF@NiCo2O4 structure on nickel foam (NF).



DOI: 10.1039/C8DT03598A

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Fig. 1 (a and b) FE-SEM images and (c) EDX pattern of NF@NCO on nickel foam. (d and e) FE-SEM images and (f) EDX pattern of NF@NMO nanostructure on nickel foam. (g and h) FESEM images and (i) EDX pattern of NF@NMO@NCO nanostructure on nickel foam. The surface morphologies of the NF@NCO nanoplates, NF@NMO honeycombs, and NF@NMO@NCO Nano composites on nickel foam were analyzed by FE-SEM, as shown in Fig. 1a–i. The low and high-magnification FESEM images of the NF@NCO indicated an orderly nanoplate morphology on nickel foam (Fig. 1(a) and (b)). Fig. 1a presents a low magnification image of the NF@NCO nanostructure. A well-ordered nanoplate morphology was observed on the surface of the Ni foam with a diameter of 60-100 nm. The high-resolution SEM image in Fig. 1b indicates a good surface area with well-ordered nanoplates. Fig. 1c presents the EDX spectrum showing that the NF@NCO nanoplates were comprised mainly of Ni, Co, and O. Fig. 1(d) and (e) present the morphology of the NF@NMO honeycomb nanostructure, showing



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almost uniform growth with a large number of pores on the nickel foam. These types of pores of honeycomb nanostructures are favorable for the easy access of electrolyte ions. EDS analysis of

typical FE-SEM images (Fig. 1g,h) of mixed metal oxides of NF@NMO@NCO were observed at different resolutions. After the chemical bath deposition process, the NF@NCO nanoplates


uniformly the surface of the NF@NMO honeycomb nanostructure decorated, forming a honeycomb with folded and silk like morphologies (Fig. 1g). This type of nanostructure supported each other to form an interconnected network morphology, which might possess remarkable mechanical capability. From Fig. 1h, it is clear that a honeycomb with a folded silklike structure, e.g., NF@NMO@NCO composites, had been well prepared using the chemical bath deposition process. The well-interconnected nanoplates on the honeycomb surface of NF@NMO@NCO provided suitable diffusion channels for the electrolyte to penetrate the electrodes and electron transportation that enables a large accessible surface area, which makes a significant contribution to enlarging the energy storage applications. EDS was also performed to confirm the formation of the NF@NMO@NCO nanostructure, as illustrated in Fig. 1i. This indicates the presence of Ni, Mo, and Co and as well as O with no other signals detected. From Fig. 1i, the elemental mapping images of Mo, Co, Ni, and O elements once again confirm the formation of NiMoO4 to NiCo2O4. Atomic percentages of Mo, Ni, Co, and O are obtained to be 9.34%, 44.30% 18.82% and 27.51%, respectively. Their atomic molar ratio is thus about 1:5:2:4, respectively.


the NF@NMO sample confirmed the presence of Ni, Mo, and O (Fig. 1f). Interestingly, the

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Fig. 2 (a, b and C) TEM and HR-TEM images of NF@NMO honey-comb structures, (d, e, and f) TEM and HR-TEM images of NF@NMO@NCO composite material. For a more detailed morphology, crystalline properties, and microstructures, the NF@NCO, NF@NMO and NF@NMO@NCO composite electrodes were characterized by TEM and high resolution TEM (HR-TEM). Fig 2(a-c) shows TEM and HR-TEM images of NF@NMO. NF@NMO delivered a crumpling silk-like nanostructure, as shown in Fig. 2a. Fig. 2b shows dark strips of a crumpling silk-like structure. The dark strips could form relatively thin curling edges. This is consistent with the interplanner spacing of 0.7 nm corresponding to the (001) planes, which is also consistent with the SEM results (Fig. 2b). In addition, the zoomed HRTEM image in Fig. 2c showed that NF@NMO exhibits a lattice fringe spacing of 0.27 nm corresponding to the (222) planes. The TEM and HR-TEM images taken on the surface of both NF@NCO nanoplates showed uniform growth along the NF@NMO honeycomb nanostructures



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(Fig. 2d-f). Fig. 2d and e shows an obvious core-shell nanostructure. In addition, the zoomed HR-TEM image in Fig. 2f shows a lattice fringe spacing of approximately 0.263 nm and 0.281

by the SEM morphology. The TEM images (Fig. S2(a-c)) of NF@NCO confirmed that NF@NCO exhibits a nanoplate-like morphology. In addition, Fig. S2c shows a HR-TEM image,


clearly resolved lattice fringes could be determined and the corresponding lattice spacing was estimated to be 0.279 nm and 0.165 nm, which corresponds to the (311) and (440) planes of the NF@NCO sample.

Fig. 3 XRD pattern of NF@NCO, NF@NMO, and NF@NMO@NCO. The purity, structure, and crystalline phase of the as-prepared NF@NCO, NF@NMO, and NF@NMO@NCO samples were examined by powder XRD, as shown in Fig. 3. The XRD patterns of NF@NCO and NF@NMO showed that the XRD peaks matched the cubic phase of NiCo2O4 (JCPDS card no. 73-1702)36, and several XRD peaks were assigned to the impurity phase of NiMoO4 (JCPDS No. 20-0781).37 The NF@NMO@NCO composite exhibited peaks at


nm, which is also in good agreement with the XRD result. The TEM images are well supported

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18.9°, 31.2°, 36.7°, 38.4°, 44.6°, 55.4°, 59.1°, and 65° 2θ, corresponding to the NiCo2O4 (111), (220), (311), (222), (400), (422), (511) and (440) planes, respectively. The characteristic crystal


60.3°, for NiMoO4 confirmed the successful growth of the material.

Fig. 4 XPS spectra: survey spectrum (a), Ni 2p (b), Mo 3d (c) and Co 2p (d) for the NF@NMO@NCO composite material. The elemental composition and chemical states of the as-synthesized NF@NMO@NCO were characterized by XPS analysis, as illustrated in Fig. 4. Fig. 4a indicates the XPS survey spectra of NF@NMO@NCO confirming the existence of Ni, Mo and Co from the reference with no other elements detected. The Ni 2p core level spectrum was reasonably deconvoluted into two peaks at 855.78 eV and 873.11 eV corresponding to Ni 2p3/2 and Ni 2p1/2, respectively, along


planes of (201) at 27°, (112) at 33.5°, (131) at 36.8°, (040) at 42°, (133) at 53°, and (440) at

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with their satellite peaks at 861.3 and 879.75 eV, as shown in Fig. 4b.38 Fig. 4c shows the XP spectra of Mo 3d, in which 232.1 and 235.2 eV were assigned to Mo 3d5/2 and Mo 3d3/2,

state.39 Fig. 4d shows, for the Co 2p core level spectrum, the peaks at 779.5 and 794.6 eV were attributed to Co 2p3/2 and Co 2p1/2.40 The shake-up satellite peaks of Co 2p appeared at 786.8 and


803.1 eV.41 In particular, the O 1s spectrum showed a unique peak at 530.1 eV, which is typical of metal-oxygen bonds, as shown in Fig. S1 in the supporting information.42 This suggests that these types of deconvoluted Ni 2p, Mo 3d, Co 2p, and O 1s peaks confirm the presence in NF@NMO@NCO, which is expected to play a key role in the electrocatalytic activity.

Fig. 5 (a) Comparative CV curves of the NF@NCO, NF@NMO and NF@NCO@NMO electrodes at a scan rate of 10 mV s-1. (b) Comparative GCD curves of the NF@NCO, NF@NMO and NF@NCO@NMO electrodes at a current density of 20 mA g-2. (c) Calculated specific capacitance at different current densities. (d, e and f) CV curves of the NF@NCO,


revealing the presence of elemental Mo in the prepared chemical particles as the Mo6+ oxidation

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NF@NMO and NF@NMO@NCO electrodes at various scan rates. (g, h and i) GCD curves of the NF@NCO, NF@NMO and NF@NMO@NCO electrodes at various current densities. electrochemical

performance

of

the

as-prepared

NF@NCO,

NF@NMO

and

NF@NMO@NCO electrodes were investigated in a three-electrode electrochemical cell with 3


M aqueous KOH solution as the electrolyte on Ni foam for application to energy storage. Fig. 5a presents the comparative CV curves of the NF@NCO, NF@NMO and NF@NMO@NCO samples measured at a scan rate of 10 mV s-1 with a potential window between -0.2 to 0.5 V. The NF@NMO@NCO nanocomposite electrode delivered a higher redox peak intensity with a larger CV integrated area than the other electrodes (NCO and NMO), suggesting much larger pseudocapacitive performance for the NF@NMO@NCO. Fig. 5b presents the comparative charge-discharge plots of NF@NCO, NF@NMO, and NF@NMO@NCO nanostructures within a potential range of 0 to 0.4 V at 20 mA g-2. Similar to the CV data, the GCD plots of the NF@NMO@NCO electrode delivered a much longer discharge time than the other electrodes, such as NF@NCO and NF@NMO, respectively. Fig. 5c shows the specific capacitance of the electrode based on the mass loading of the materials. The NF@NMO@NCO electrode exhibited a high specific capacitance value of 2695 F g-1 at 20 mA g-2, which is larger than those of NF@NCO (1018 F g-1) and NF@NMO (1194 F g-1). Furthermore (Fig 5d-f), all the CV curves delivered a pair of strong redox peaks over the entire range of scan rates, demonstrating the desirable pseudocapacitive behavior of the electrode and rapid ionic transport. With increasing scan rates, the anodic and redox current intensity also increased and similar CV curve shapes were observed due to the strengthened electrical polarization of the electroactive materials.43 Fig. 5d shows the CV curves of the NF@NCO electrode at scan rates of 5, 10, 15, and 20 mV s-1 over the potential window of -0.2 to 0.5 V. Fig. 5e presents the CV curves of the NF@NMO


The

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electrode material within a potential range of -0.2 to 0.5 V at different current densities, such as 5, 10, 15 and 20 mV s-1. Fig. 5f shows the CV curves of NF@NMO@NCO recorded at the same

NF@NCO electrode materials, which exhibits specific capacitances of 1018, 750, 570, 480, and 455 F g-1 and NF@NMO delivered specific capacitances of 1194, 1150, 900, 650, and 585 F g-1


at the current densities of 20, 22, 24, 26, and 28 mA g-2, respectively. Interestingly, the NF@NMO@NCO electrode exhibited significantly enhanced specific capacitance of 2695, 2605, 2541, 1891, and 1527 F g-1 at the same current densities.


scan rates and potential range. Fig. 5g shows the specific capacitance estimated from the

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Fig. 6 (a) EIS curve comparison between NF@NCO, NF@NMO, and NF@NMO@NCO. (b) Ragone plot (energy density vs. power density curve) of NF@NMO@NCO. (c) Cycling

image).


Furthermore, EIS was carried out to examine the conductivity of the resulting materials NF@NCO, NF@NMO, and NF@NMO@NCO electrodes, and Fig. 6a shows the Nyquist plot. Basically, a semicircle in the high frequency region, corresponding the charge-discharge resistance (Rct), and the slope of curves in low frequency area reflects the electrolyte diffusion in the active electrode materials.44 In particular, from the figure, it is clear that the NF@NMO@NCO (0.284 Ω) electrode exhibited an ideal straight line in the lower frequency area than those of NF@NCO (0.302 Ω) and NF@NMO (0.501 Ω), which demonstrates the lowest ion diffusion resistance, rapid electron transfer kinetics, and confirmed better conductivity. The NF@NMO@NCO electrode exhibited the lowest Rct. This suggest that the NF@NMO@NCO nanostructure exhibited a lower charge transfer resistance with a short ion diffusion path and rapid faradaic process with larger power. Fig. 6b presents a Ragone plot between the energy density and power density values of the supercapacitor

were

gained

from

the

charge-discharge

curves.

The

as-synthesized

NF@NMO@NCO electrode achieved an excellent energy density of 61.2 W h kg-1 at a power density of 371.5 W kg1. The achieved energy density of this study presented superior or comparable performance to those of previously reported three-electrode configuration based on a composite materials, such as 52.6 W h kg-1 (NiCo2O4 UNSA@NiMoO4//AC),45 23.3 W h kg-1 (NiCo2O4-RGO//AC),46

21.7

W

h

kg-1

(NiCo2O4@MnO2//AC),4735.2

(Ni0.75Co0.25MoO4//AC),48 and 28 W h kg-1 (β-NiMoO4-CoMoO4.xH2O//AC).49

W

h

kg-1


performance of the NMO@NCO electrode up to 3000 cycles at 22 mA g-2 (inset shows the SEM

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In addition, the cycling stability, which is an important factor in determining the practical applicability of the NF@NMO@NCO electrode, was also evaluated by repeated charge-

electrode system, as shown Fig. 6c. Importantly, the cycling test showed that the NF@NMO@NCO electrode had excellent capacitance retention, exhibiting 98.9% of the initial


value even after 3000 cycles, which is superior to those reported elsewhere: 90.6% initial value after 2000 cycles (NiCo2O4@NiMoO4),50 83% of the initial capacitance after 2000 cycles (NiMoO4@CoMoO4),51 89% capacitance retention after 3000 cycles (Ni0.75Co0.25MoO4//AC),52 89.4% retention after 2000 cycles (NiCo2O4-MnO2/GF//CNT/GF),53 and 90% capacitance retention after 2000 cycles (α-NiCo2O4@MnO2//NC).54 After the cycling test, the SEM image of NF@NMO@NCO (the inset of Fig. 6c) showed that the nanoplate’s structure of NCO electrode and honeycomb structure of NF@NMO electrode had been retained in the NF@NMO@NCO composite, suggesting that there is no observable change in the morphology of the sample. The good cycling stability of the NF@NMO@NCO electrode was attributed to the high structural stability of the electrode that could be provide more favorable transport channels for electrons and more active redox reactions in the electroactive materials for higher activation of the materials.

Conclusion In summary, NF@NMO@NCO honeycomb and folded with silk-like nanostructure were fabricated successfully on Ni foam using a facile and cost-effective chemical bath deposition process. The NF@NMO@NCO material was characterized using a variety of physical-chemical techniques. In a three-electrode system, the NF@NMO@NCO electrode exhibited a superior specific capacitance of 2695 F g-1 at 20 mA g-2, which was much higher than those of NF@NCO


discharge measurements at a constant current density of 22 mA g-2 for 3000 cycles in a three-

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(1018 F g-1) and NF@NMO (1194 F g-1) due to the NF@NCO nanoplates on NF@NMO honeycomb superstructures exhibiting more favorable transport channels for electrons, larger

conductivity. Moreover, the high capacitance material NF@NMO@NCO exhibited an excellent energy density of 61.2 W h kg-1 and higher power density of 371 W kg-1. In addition, the


NF@NMO@NCO material delivered superior cycling stability with 98.9% retention after 3000 cycles at a current density of 22 mA g-2, which was comparable to previously reported supercapacitors. In view of the facile and cost-effective fabrication and the attractable energy storage performance, the NF@NMO@NCO honeycomb folded with a silk like nanostructure has great potential as an advanced electrode material for high-performance supercapacitor applications.

Conflicts of interest There are no conflicts to declare.

Acknowledgements This research was supported by the Basic Research Laboratory through the National Research Foundations of Korea funded by the Ministry of Science, ICT and Future Planning (NRF2015R1A4A1041584). In addition, this work was financially supported by BK 21 PLUS, Creative Human Resource Development Program for IT Convergence, Pusan National University, Busan, South Korea. Finally, special thanks are due to the KBSI for the instrumental characterization.

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Scheme. 1 Schematic representation of the synthesis of honeycomb with folded silk-like NF@NiMoO4@NiCo2O4 nanostructure, honeycomb-like NF@NiMoO4 structure and nanoplatelike NF@NiCo2O4 structure on nickel foam (NF).



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