Facile hydrothermal synthesis of NiCo2O4- decorated filter carbon as electrodes for high performance asymmetric supercapacitors

Accepted Manuscript Facile hydrothermal synthesis of NiCo2O4- decorated filter carbon as electrodes for high performance asymmetric supercapacitors Guijun Yang, Soo-Jin Park PII:

S0013-4686(18)31781-X

DOI:

10.1016/j.electacta.2018.08.013

Reference:

EA 32447

To appear in:

Electrochimica Acta

Received Date: 25 June 2018 Revised Date:

26 July 2018

Accepted Date: 4 August 2018

Please cite this article as: G. Yang, S.-J. Park, Facile hydrothermal synthesis of NiCo2O4- decorated filter carbon as electrodes for high performance asymmetric supercapacitors, Electrochimica Acta (2018), doi: 10.1016/j.electacta.2018.08.013. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

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Manuscript submitted to "Electrochimica Acta" as a research paper.

Facile hydrothermal synthesis of NiCo2O4- decorated filter carbon as electrodes for high

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performance asymmetric supercapacitors

Guijun Yang and Soo-Jin Park*

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Department of Chemistry, Inha University, 100 Inharo, Incheon, Korea

*Corresponding author. Tel.: +82-32-876-7234, Fax: +82-32-867-5604.

E-mail addresses:[email protected] (S. -J. Park)

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Abstract A simple hydrothermal method is developed for the growth of NiCo2O4 nano-needle

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arrays on a filter paper carbon (FC) substrate for use as a high performance electrode for supercapacitors. The as-synthesized NiCo2O4/C composites exhibited high specific

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capacitance (995.2 F g-1 at a current density of 10 A g-1.) and excellent cycling

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performance (no capacitance loss after 3000 cycles). The 3D framework of the FC substrate not only prevents the aggregation of NiCo2O4 nanoparticles, but also improves its electrical conductivity, thus leading to excellent electrochemical performance. Moreover, an asymmetric supercapacitor was assembled by using the NiCo2O4/C

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composites and activated carbon as the positive and negative electrode, respectively, and provided a maximum energy density of 20.87 Wh kg-1 at a power density of 374.6

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W kg-1 and a maximum power density of 7.48 kW kg-1 at an energy density of 11.43 Wh

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kg-1 within a potential window of 0-1.5 V, demonstrating the potential of the composites for use in high performance energy storage systems.

Keywords:

NiCo2O4/C

composites,

electrochemical performance.

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nano-needle

arrays,

supercapacitor,

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1. Introduction With the rapid development of society and economy worldwide, finding solutions

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to the problems of energy shortages and environmental pollution has become urgent. In recent years, great efforts have been made to develop clean and renewable energy, with

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the storage of energy in a cheap, rapid, and efficient way being a major focus of

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study.[1, 2] Supercapacitors (SCs) combine the fast charging and discharging characteristics of traditional capacitors and the energy storage characteristics of batteries, and represent a new type of energy storage devices intermediate between batteries and electrostatic capacitors. Due to their high power density, high discharge

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rate, and long cycle life, supercapacitors have become a hot topic in energy storage research. Supercapacitors are generally divided into two types: electrical double layer

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capacitors (EDLCs), which utilize carbonaceous materials as electrodes,[3-5] and

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pseudocapacitors, which utilize transition metal oxides or conducting materials.[6-8] Transition metal oxides such as RuO2,[9, 10] NiO,[11, 12] Co3O4,[13, 14] MnO2,[15, 16] V2O5,[17, 18] and TiO2[19, 20] have high specific capacitance due to the rapid reversible redox reactions on the surface of electrodes. However, most of these materials have the drawbacks of high raw material costs, poor electrical conductivity, and poor rate capability. The transition metal oxide NiCo2O4 has attracted widespread 3

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attention due to its abundance, good electrochemical performance, and environmental friendliness. Recently, NiCo2O4

has been widely applied in the application of

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supercapacitors, Lithium-sulfur batteries and Li-ion batteries.[21-23] In the spinel structure of NiCo2O4 oxide, Ni2+ is introduced into the lattice of Co3O4 by substituting

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Ni for tetrahedral Co2+ and octahedral Co3+. Co-Ni compounds have potential

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advantages in electronic devices owing to its excellent properties. The Ni2+/Ni3+ and Co2+/Co3+ redox reactions are responsible for the high capacitance of NiCo2O4.[24-26] However, nanosize NiCo2O4 inevitably undergoes agglomeration, and its electronic conductivity is low; its specific capacitance also decreases rapidly at high current

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densities, which results in poor electrochemical performance. In order to obtain excellent performance of NiCo2O4 material, it depends on not only the area of contact

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between the electrolyte and the electrode materials, but also the transport rates of

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ions/electrons in the electrode and at the electrode/electrolyte interface. Therefore, it is a good strategy to combined active material with a conductive substrate with porous structure, large specific surface area and high conductivity. Carbonaceous materials such as graphene,[27, 28] carbon fibers,[29, 30] and actived carbon[31, 32] has been widely applied to the energy storage system. These kind of substrate can bring many advantages, such as abundant electroactive sites, short diffusion path and good 4

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conductivity, which provides excellent electrochemical performance. However, most carbon sources for electrodes are limited natural resource, which is expensive and

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polluting. For sustainable development, activated carbon derived from biomass materials such as banana peel,[33] cotton textile[34] and paper[35] have attracted

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people’s attention. For example, Zhang et al. [36] deposited urchin-like NiCo2O4 on the

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activated carbon derived from waste banana peels (ABP) to form electrode materials for supercapacitors and batteries. The ABP provides an interconnected porous framework for NiCo2O4 nanowire, inhibiting aggregation and increasing electrolyte accessibility, thus leads to excellent electrochemical performance. Bao et al.[37] demonstrated a

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flexible activated carbon fibers (ACT) by a simple chemical activation of cotton T-shirt textile. The activated cotton T-shirt textile with porous structure shows high specific area

and

conductivity.

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surface

Furthermore,

the

NiCo2O4@NiCo2O4/ACT

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pseudocapacitors exhibited remarkable electrochemical performance (1929 F g-1, based on the mass of NiCo2O4), energy density (83.6 Wh kg-1), power density (8.4 kW kg-1), cycling stability and mechanical robustness. This kind of design provides a low-cost and environment friendly way to energy storage systems. In this study, we demonstrate a simple hydrothermal approach to obtain a filter paper derived carbon-based NiCo2O4 material for use in supercapacitors. In this 5

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process, filter paper was submerged in a NiCo2O4 precursor solution, and the final product was obtained after calcination in an Ar atmosphere. The preparation process

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of NiCo2O4 and the NiCo2O4/C composites is shown in Scheme 1. The hydroxyl group-containing cellulose in the filter paper facilitates the formation of 3D structures,

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which not only improve the electrical conductivity but also provide fast electron and

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ion transport after calcination at high temperature, thus improving the electrochemical performance.

2. Experimental

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2.1. Preparation of NiCo2O4 and NiCo2O4/C composites NiCo2O4/C composites were synthesized using a hydrothermal method, and the

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preparation process is shown in Scheme 1. In a typical process, 0.293 g of nickel nitrate

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hexahydrate (Ni(NO3)2·6H2O, 99%, Aldrich), 0.146g of cobalt nitrate hexahydrate (Co(NO3)2·6H2O, 99%, Aldrich), and 0.909 g of urea (CO(NH2)2, 99%, Aldrich) were dissolved in deionized water and stirred for 30 min. The solution was then transferred to a 100 mL Teflon-lined autoclave and maintained at 140 °C for 3 h. The precursor was subsequently washed with deionized water several times and dried at 60 °C for 12 h. Finally, the dried precursors were calcinated at 250, 300, 350, or 400 °C for 2 h under 6

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an Ar atmosphere. The obtained products were labeled as NCO-250, NCO-300, NCO350, and NCO-400. For comparison, different contents of filter paper (0.1 g, 0.2 g, or

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0.3 g) were added to the aqueous solution and stirred for another 30 min. Precursors with different filter paper contents were then synthesized by the same process. Using the

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procedure described above, the precursors were calcinated at 350 °C for 2 h. The

2.2. Characterization

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obtained composite materials were labeled as NCO/C-1, NCO/C-2, and NCO/C-3.

The obtained samples were characterized using X-ray diffraction (XRD, Bruker D2 PHASER, with Cu Kα), scanning electron microscopy (SEM, Model SU8010, Hitachi

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Co., Ltd.), Energy dispersive X-ray measurements were conducted using the EDAX system attached to the same microscope, field-emission transmission electron

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microscopy (FE-TEM, JEOL JEM-2100F), X-ray photoelectron spectroscopy (XPS,VG

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Scientific Co., ESCA LAB MK-II). The specific surface areas (SSA) of the composites were investigated by the Brunauer-Emmett-Teller (BET) method at 77 K. The pore size distributions were calculated by the density functional theory (DFT) model. The total pore volume was determined at a relative pressure of P/P0=0.99.

2.3. Electrochemical measurements 7

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The working electrode was prepared by coating a slurry of the active material (80 wt.%), ketjen black (electronic conductive additive, 10 wt.%), and poly vinylidene

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fluoride (PVDF, binder, 10 wt.%) in the solvent N-methyl pyrrolidone (NMP) onto a nickel foam. The obtained electrodes were then dried at 60 °C overnight under vacuum.

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Typically, the mass loading of active material was around 1.5 mg cm-1. As three-

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electrode systems mainly depend on the electrochemical performance of the electrode material and cannot reflect the performance of a capacitor, asymmetric supercapacitors were also assembled to test the electrochemical performance of the device. The threeelectrode system consisted of the prepared NiCo2O4 materials as the working electrode,

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a platinum (Pt) wire as the counter electrode, and a saturated calomel electrode (SCE) as the reference electrode. In the two-electrode configuration, filter paper and activated

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carbon were used as the separator and counter electrode, respectively. The electrolyte

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was a 6 M KOH aqueous solution. Cyclic voltammetrytesting (CV), galvanostatic charge-discharge (GCD) measurements and electrochemical impedance spectroscopy (EIS) measurements were performed using an Ivium electrochemical workstation at room temperature.

3. Results and discussions 8

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The effect of different calcination temperatures on NiCo2O4 was investigated using XRD measurements. As shown in Fig. S1a in the supporting information, no diffraction

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peaks were observed below 250°C. The diffraction peaks at 36.6°, 44.7°, and 63.9°, which are characteristic of NiCo2O4, appeared in the XRD pattern at 300 °C, and the

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crystallinity of the NiCo2O4 crystals improved with increasing calcination temperature.

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Fig. S1b shows the galvanostatic charge/discharge curves of NCO-300, NCO-350, and NCO-400 at a current density of 2 A g-1. The NCO-350 sample exhibited the longest discharge time, which indicated that the NCO-350 sample had the best discharge

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performance. Therefore, we chose 350 °C as the optimal temperature in this experiment.

Fig. 1 shows the XRD patterns of the NiCo2O4/C composites with different

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contents of filter paper derived carbon (FC). From the XRD profile, we can see the diffraction peaks at 36.6°, 44.7°, and 63.9° corresponding to the (3 1 1), (4 0 0), and (4 4

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0) planes of NiCo2O4 (JCPDS Card No. 20-0781). In the XRD patterns of the NiCo2O4/C composites, another sharp diffraction peak corresponding to carbon appeared at around 23°, indicating the successful synthesis of the NiCo2O4/C composites.

XPS measurements were carried out to analyze the oxidation state of the NiCo2O4 9

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and NiCo2O4/C composites. The XPS spectra were corrected using the C 1s level (284.8 eV). The XPS survey spectra in Fig. 2a shows the existence of the elements Ni, Co, and

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O in the two as-synthesized samples. There is an obvious C1s peak in the spectra of the NiCo2O4/C composites due to the filter paper carbon additives. However, a small

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amount of carbon was also observed in the NiCo2O4 sample because of the long-term

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exposure of the sample to air. The high resolution XPS Ni 2p, Co 2p, and O 1s spectra are shown in Fig. 2b-d, respectively. The Ni 2p emission spectrum (Fig. 2b) was consistent with two spin-orbit doublets (Ni2+ and Ni3+) and two shakeup satellites (marked as "Sat."). For the NiCo2O4 sample, the fitting peaks at 855.7 and 873.8 eV

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correspond to Ni2+, while the peak at 856.9 eV corresponds to Ni3+. The satellite peaks at 861.2 (Ni 2p3/2), 864.7 (Ni 2p3/2), and 879.9 eV (Ni 2p1/2) are three shakeup-type

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nickel peaks. For the NiCo2O4/C composites, the fitting peaks at 855.7, 856.3, and

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873.8 eV correspond to Ni2+, while the peaks at 854.4 and 871.8 eV correspond to Ni3+. The satellite peaks at 861.2 (Ni 2p3/2), 864.7 (Ni 2p3/2), and 879.9 eV (Ni 2p1/2) are three shakeup-type nickel peaks.[38-40] The Co spectrum (Fig. 2c) was also fitted with two spin-orbit doublets (Co2+ and Co3+) and two shakeup satellites. The fitting peak of the NiCo2O4 sample at 783.0 eV corresponds to Co2+, while the peaks at 781.3 and 797.1 eV correspond to Co3+. The satellite peaks at 787.3 and 803.0 eV are two shakeup-type 10

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cobalt peaks. The fitting peaks of the NiCo2O4/C composites at 778.7 and 782.6 eV correspond to Co2+, and the peaks at 780.4 and 782.6 eV correspond to Co3+. The

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satellite peaks at 786.8 and 803.0 eV are two shakeup-type cobalt peaks.[41, 42] The high-resolution spectrum of the O 1s region of the NiCo2O4 sample (Fig. 2d) shows

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three oxygen contributions. The peaks observed in the spectra at 529.7, 531.2, and 532.6

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eV can be ascribed to metal-oxygen bonds, defect sites with low oxygen coordination at the surface, and the hydroxyl species of adsorbed water on the surface of the sample, respectively.[43, 44] For the NiCo2O4/C composites, in addition to the peaks at 529.7 and 531.2 eV, a peak is observed at 531.8 eV and was ascribed to O-C/O=C bonds on

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the surface of carbon.[45] The XPS results indicated that the Ni and Co in the asprepared NiCo2O4 and NiCo2O4/C composites were mixed valence (Ni2+, Ni3+, Co2+ and

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    Co Co3+).The chemical formula can be written as Co  Co Ni Ni O 0 ≤ x ≤

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1, which is in agreement with previous reports.[46]

The porous textures of NiCo2O4 and NiCo2O4/C composites were characterized

by N2 adsorption-desorption at 77 K. As shown in Fig. S2, All the isotherm profiles exhibits a type IV isotherm with a significant hysteresis loop at a relative pressure between 0.45 and 0.95, indicating that the NiCo2O4 and NiCo2O4/C composites have a 11

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typical mesoporous structure, which can be further supported by the BJH pore size distribution. The measured BET specific surface areas of NiCo2O4 and NiCo2O4/C

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composites are 73.8 and 187.81 m2 g-1, respectively. The improved specific surface area indicates that the introduction of filter carbon can reduce the aggregation of

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NiCo2O4 particles. The pore size distribution of pure NiCo2O4 and NiCo2O4/C

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composites are centered at around 5-20 nm and 2-10 nm, respectively. The N2 adsorption-desorption results show that mesoporous structures have been formed for the NiCo2O4/C composites. It can be believed that the electrode materials with high

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surface area and weak agglomeration will good for the electrochemical performance.

The morphology of NiCo2O4 (NCO-350) and the NiCo2O4/FC composites with

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different contents of FC (NCO/C-1, NCO/C-2, and NCO/C-3) were investigated using scanning electron microscopy (SEM). As shown in Fig. 3a, the NiCo2O4 sample

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exhibited a regular sea urchin shape with an average diameter of approximately 5µm. When the FC was added, the NiCo2O4 particles began to gather on the surface of the FC. Due to the lower amount of FC added in NCO/C-1 sample, a large number of NiCo2O4 particles are agglomerated on the FC, and many unattached NiCo2O4 particles are present in the NCO/C-1 sample. The diameter of the NiCo2O4 particles in 12

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the NCO/C-1sample decreased to 3 µm. As the amount of FC carbon additive was increased, a neatly arranged nano-needle array formed on the FC carbon, and no

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obvious single NiCo2O4 particles were observed in the NCO/C-2 and NCO/C-3 samples. In contrast, the morphologies of NCO/C-2 and NCO/C-3 samples were

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similar despite the increased FC content of the NCO/C-3 sample. However, the use of

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too much FC may affect the electrochemical performance of the NiCo2O4/C composites. Furthermore, the EDS analysis was carried out to confirm the detailed elemental composition of the NCO/C-2 sample as demonstrated in the inset of Fig. 3c. It can be seen that the NCO/C-2 sample contains C, O, Co and Ni elements, and the

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atomic ration of C: O: Co: Ni = 73.1%: 19.5%: 4.3%: 3.0%.

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The structural characteristics of the filter carbon, NCO-350, and the NiCo2O4/C composites were further investigated using TEM. As shown in Fig. 4a, the filter carbon

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consists of nanorods with a length of approximately 200 nm and a diameter of approximately 10 nm. Fig. 4b shows the uniform size and typical sea urchin shape of the NCO-350 sample, which agrees well with the SEM results. The TEM images the NiCo2O4/C composite is shown in Fig. 4c, in which the NiCo2O4 nano-needle array is neatly arranged on the filter carbon. The conductive carbon substrate and clear array 13

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structure of the NiCo2O4/C composites resulted in good electrochemical properties. The corresponding elemental mapping images are shown in Fig. 4d, and further confirmed

distributed throughout the area marked with a red rectangle.

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the structure of the NiCo2O4/C composites, in which all the elements are uniformly

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The electrochemical performances of NCO-350 and the NiCo2O4/C composites

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were first studied using cyclic voltammetry. Fig. 5 shows the CV curves of NCO-350 and the NiCo2O4/C composites (NCO/C-1, NCO/C-2, and NCO/C-3) at various scan rates from 5-30 mVs-1 in a 6 M KOH electrolyte. The CV curves of NiCo2O4exhibit obvious Ni2+/Ni3+ and Co2+/Co3+ redox peaks, which are distinct from the electric

are as follows:[47]

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double layer capacitance. The equations of these redox reactions in an alkaline solution

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NiCo O + OH  + H O ↔ NiOOH + 2CoOOH + e

(1)

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CoOOH + OH  ↔ CoO + H O + e (2)

As the scan rate increased, the area of the CV curves also increased, indicating a

faster transport rate of electrons and ions, which in turn corresponds to relatively rapid surface redox reactions on the surface of the electrode. In contrast, the redox potential difference of NCO-2 was relatively small, indicating smaller polarization and better reversibility during the redox process. 14

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Galvanostatic charge-discharge (GCD) measurements were carried out to further study the electrochemical performance of the as-synthesized electrodes. As shown in

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Fig. 6a-d, the GCD curves of NCO-350, NCO/C-1, NCO/C-2, and NCO/C-3 were tested within a potential window from 0 to 0.4 V at different current densities (1, 2, 5,

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and 10 A g-1). The plateaus in the voltage indicate the existence of faradaic processes,

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verifying the pseudocapacitive behavior of the electrodes. The specific capacitances were calculated from the GCD curves using the following equation: 

C = /!

(3)

Where Cs is the specific capacitance (F g-1), I is the constant discharge current (A),

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m is the mass of the active material (g), dV is the potential window (V), and dt is the discharge time (s), respectively. The calculated specific capacitances of the electrodes

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are plotted in Fig. 6e. The specific capacitance of the electrodes decreased with

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increasing current density, due to the insufficient time for ion diffusion at high current density. Among the four as-prepared samples, the NCO/C-2 sample exhibited the longest discharge time, indicating its higher discharge capacitance and better rate capability. The capacitance of NCO/C-2 decreased from 1480.9 to 995.2 F g-1as the current density increased from 1 to 10 A g-1, retaining 67.2% of its specific capacitance, while the NCO-350, NCO/C-1, and NCO/C-3 samples retained only 23.7%, 24.6%, and 15

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35.0%, respectively. The excellent electrochemical performance of NCO/C-2 was due to the FC carbon substrate, which can efficiently improve electrical conductivity and

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reduce the agglomeration of NiCo2O4 nanoparticles. The lower specific capacitance of NCO/C-3 was due to the excess bio-char, which blocked the transport paths of the

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electrolytes and ions, leading to decreased power density. In addition, the long term

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cycling performance of the NCO/C-2 sample is shown in Fig. 6f. The NCO/C-2 electrode exhibited excellent cycling performance during 3000 cycles at a scan rate of 100 mV s-1 without loss of capacity, indicating its excellent cycle stability. To further examine the resistance behavior of electrodes, the EIS analysis of the

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samples have been carried out In the high-frequency region, the real axis intercept represents the solution resistance (Rs). The Nyquist plot is composed of a semicircle at

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the high- to intermediate-frequency range and a straight line in the low-frequency range,

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corresponding to the charge transfer resistance (Rct) and capacitance behavior, respectively. Fig. S3 shows the Nyquist plots of pure NiCo2O4 and NiCo2O4 with different FC amounts. The NCO/C electrodes show lower Rs than pure NiCo2O4, indicating that the diffusion resistance between electrolyte and active material were decreased by the introduction of FC and the good desperation of NiCo2O4. Moreover, the Rct of NiCo2O4/C composites are lower than that of pure NiCo2O4, suggesting that 16

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the electrical conductivity and charge transfer rate of the electrodes are also improved by the introduction of FC. In addition, the slopes of straight lines at low frequency

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region gradually grow larger with the increase of FC, manifesting typical capacitor behavior of these samples.

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As three-electrode systems mainly reflect the electrochemical performance of an

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electrode material, and cannot reflect capacitor performance, an asymmetric supercapacitor (ASC) was also assembled to test the electrochemical performance of the devices. The aqueous asymmetric supercapacitor was assembled using NCO/C-2 as the positive electrode and activated carbon (AC) as the negative electrode (denoted as

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NCO/C-2//AC). The assembly diagram of the NCO/C-2//AC device is shown in Fig.7a. The mass ratio of the positive and negative electrode was calculated based on charge

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balance theory (q+=q-); the equation can be written as follows: (4)

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C ∆#  m = C ∆#  m

Where Cs+ and Cs-are the specific capacitances of the NCO/C-2 and AC electrodes

(F g-1), ∆V is the potential window (V), and m+ and m-are the masses of NCO/C-2 and AC electrode (g), respectively. Fig. 7b presents the CV curves of the activated carbon and NCO/C-2 in 6 M KOH solution at a scan rate of 20 mV s-1. According to eq.(4) above and Fig. 7b, the optimal mass ratio between the NCO/C-2 and AC electrodes was 17

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m+/m-=0.24. The potential window of activated carbon ranged from -1.0 to 0 V, while that of NCO/C-2 ranged from 0 to 0.6 V, indicating a wide potential window for the

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NCO/C-2//AC device, ranging from 0 to 1.6 V. Fig. 7c shows the CV curves of the ASC device for different potential windows at a scan rate of 100 mV s-1. An obvious hump is

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observed for a potential window of 1.6 V, suggesting that some irreversible oxidation

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reactions occurred; thus, the stable working potential window of the NCO/C-2//AC device is from 0 V to 1.5 V. Fig. 7d shows the CV curves of NCO/C-2//AC with increasing scan rates from 20 mV s-1 to 100 mV s-1 in 6 M KOH aqueous solution. The CV curves show obvious redox peaks, corresponding to the redox reaction of NCO/C-2,

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and the CV curves exhibit a quasi-rectangular shape, indicating that both double-layer capacitance and pseudocapacitance contribute to the capacitance of the NCO/C-2//AC

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device. As the scan rate increased, the shape of CV curves did not change significantly,

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implying that the electrodes had good electrochemical stability, and the area of the CV curves increased obviously, demonstrating the good rate capability of the ASC devices. Fig. 8a shows the galvanostatic charge/discharge curves of NCO/C-2//AC ASC

device at various current densities. The GCD curves were approximately triangular as the discharge rate increased. The NCO/C-2//AC device exhibited symmetric and linear charge/discharge curves with a small plateau, which provided further evidence that the 18

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capacitance of NCO/C-2//AC device was the result of mixed capacitance. The specific capacitance of the NCO/C-2//AC device was calculated according to eq. (3) and is

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plotted in Fig. 8b. The discharge specific capacitance of the ASC device was 66.8 F g-1 at a current density of 0.5 A g-1, while it decreased to 36.5 F g-1 at 10 A g-1, retaining

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54.6% of the initial capacity. The Ragone plot of the ASC device is shown in Fig. 8c. The energy density and power density were calculated using the following equations:



∆)

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E = C ∆V  P =



(5)

Where E is the energy density (Wh kg-1), P is the power density (W kg-1), Cs is the specific capacitance (F g-1), ∆V is the potential window (V), I is the constant discharge

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current (A), and m is the mass of the active material (g). The ASC device showed a maximum energy density of 20.87 Wh kg-1 (at a power density of 374.6 W kg-1) and

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maximum power density of 7.48 kW kg-1 (at an energy density of 11.43 Wh kg-1). Its

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performance exceeded the previous reports listed in Table 1.[54-57] In addition, Fig. 8d presents the cycling performance of the ASC device during 5000 cycles at a scan rate of 100 mV s-1. Its specific capacitance maintained 83.04% of the initial capacitance value after 5000 cycles at a high scan rate. Therefore, the as-designed NCO/C-2//AC ASC device has potential application prospects in energy storage devices.

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4. Conclusion In summary, NiCo2O4/C composites with filter paper carbon as the substrate have

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been synthesized through a simple hydrothermal approach for use in supercapacitors. The filter paper carbon with 3D structures can improve the electrical conductivity of

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NiCo2O4/C composites and reduce the agglomeration of the NiCo2O4 nanoparticles.

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Moreover, the 3D framework with open channels can increase the electron and ion transport rates, which can improve the electrochemical performance of the NiCo2O4/C composites. The NiCo2O4/C composites exhibit excellent cycling performance with no loss after 3000 cycles at a scan rate of 100 mVs-1, and retain a good rate capability of

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995.2 F g-1 even at 10 A g-1. In addition, an asymmetric supercapacitor has been assembled by using NCO/C-2 and activated carbon as the positive and negative

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electrodes. The NCO/C-2//AC ASC device exhibits a maximum energy density of 20.87

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Wh kg-1 at a power density of 374.6 W kg-1 and a maximum power density of 7.48 kW kg-1 at an energy density of 11.43 Wh kg-1.The device also shows good cycling performance, with 90.94% retention over 3000 cycles at 100 mV s-1. NCO/C-2//AC is expected to be a promising electrode material for energy storage devices with high energy density and power density.

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Conflicts of interest There are no conflicts to declare.

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Acknowledgements

This research was supported by the Leading Human Resource Training Program of

by

the

Ministry

of

Science,

ICT

and

future

Planning

(NRF-

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funded

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Regional Neo industry through the National Research Foundation of Korea (NRF)

2016H1D5A1909732). This work was also supported by the Technology Innovation Program (or Industrial Strategic Technology Development Program (10080293, Development of carbon-based non phenolic electrode materials with 3000 m2/g grade

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surface area for energy storage device) funded by the Ministry of Trade, Industry &

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Energy (MOTIE, Korea).

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polycrystalline cobalt oxide, J. Electron Spectrosc. 71 (1995) 61.

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[46] Y. Zhang, B. Wang, F. Liu, J. Cheng, X.-w. Zhang, L. Zhang, Full synergistic contribution of electrodeposited three-dimensional NiCo2O4@MnO2 nanosheet networks electrode for asymmetric supercapacitors, Nano Energy 27 (2016) 627.

[47] Q. Zhou, X. Wang, Y. Liu, Y. He, Y. Gao, J. Liu, High rate capabilities of NiCo2O4-based hierarchical superstructures for rechargeable charge storage, J. Electrochem. Soc. 161 (2014) A1922. [48] M. Kuang, Z.Q. Wen, X.L. Guo, S.M. Zhang, Y.X. Zhang, Engineering firecracker-like beta-

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manganese dioxides@ spinel nickel cobaltates nanostructures for high-performance supercapacitors, J. Power Sources 270 (2014) 426.

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248.

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electrochemical capacitors, Electrochim. Acta 107 (2013) 494. [51] S. Wen, Y. Liu, F. Zhu, R. Shao, W. Xu, Hierarchical MoS2 nanowires/NiCo2O4 nanosheets supported on Ni foam for high-performance asymmetric supercapacitors, Appl. Surf. Sci. 428 (2018) 616.

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Figure captains Figure 1. (a) XRD patterns of NiCo2O4/C with different content of filter paper derived

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carbon (0, 0.1, 0.2 and 0.3 g). Figure 2. XPS spectra for NiCo2O4 and NiCo2O4/C composites: (a) survey spectra (b) Ni 2p spectra, (c) Co 2p spectra and (d) O 1s.

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Figure 3. SEM images of (a) NiCo2O4 and NiCo2O4/C composites with different content of filter carbon (b) NCO/C-1, (c) NCO/C-2 NCO/C-2 (inset: the corresponding

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EDS spectrum) and (d) NCO/C-3 at different magnifications.

Figure 4. TEM images of (a) filter carbon, (b) NiCo2O4 (c) NiCo2O4/C composites and (d) elemental mapping images of an individual fiber of NiCo2O4/C composites. Figure 5. Cyclic voltammetry curves of (a) pure NiCo2O4 (NCO-350) and NiCo2O4/C

KOH aqueous solution.

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composites: (b) NCO/C-1, (c) NCO/C-2 and (d) NCO/C-3 at various scan rates in 6 M

Figure 6. The galvanostatic charge-discharge curves of (a) NCO-350, (b) NCO/C-1, (c)

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NCO/C-2, (d) NCO/C-3 at various current densities, (e) Specific capacitance calculated from GCD curves versus current densities and (f) cycling performance of NCO/C-2

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during 3000 cycles at scan rate of 100 mV s-1. Figure 7. (a) Schematic of the assembled NCO/C-2//AC asymmetric supercapacitor and cyclic voltammetry curves of (b) activated carbon and NCO/C-2 in 6 M KOH solution at a scan rate of 20 mV s-1, (c) NCO/C-2//AC device with different potential window at a scan rate of 100 mV s-1, (d) NCO/C-2//AC at various scan rates. Figure 8. Electrochemical performances of NCO/C-2//AC ASC device: (a) the galvanostatic charge-discharge curves at various current densities; (b) Specific 26

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capacitances calculated from GCD curves; (c) Ragone plot; (d) Cycling performance during 5000 cycles at a scan rate of 100 mV s-1. Scheme 1. Schematic illustration for the preparation of NiCo2O4 and NiCo2O4/C

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composites.

Figure SI 1. (a) XRD patterns of NiCo2O4 at different calcination temperature (250,

350 and NCO-400 at 2 A g-1.

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300, 350, 400 oC) and (b) Galvanostatic charge/discharge curves of NCO-300, NCO-

Fig. S2 Nitrogen adsorption-desorption isotherms of (a) NiCo2O4 and (b) NiCo2O4/C-2

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measured at 77 K (inset: pore size distribution).

Fig. S3 EIS curves (Nyquist plots) of the pure NiCo2O4 and NiCo2O4/C composites. Table 1. Energy density and power density of different devices based on NiCo2O4

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electrodes.

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Table 1 Energy density and power density of different devices based on NiCo2O4 electrodes.

Energy density

Ref.

9.4

2500

[48]

42.5

80

[49]

6.8

2805

[50]

18.4

1200.2

[51]

20.87 11.43

374.6 7480

This work

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Devices

Corresponding power density (W kg-1)

(Wh

NiCo2O4-MnO2//AG ASC

NiCo2O4 //AC ASC

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MS/NCO//AC ASC

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NiCo2O4@Co3O4 //AC ASC

AC C

NCO/C-2//AC ASC

kg-1)

AC C

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Scheme 1. Schematic illustration for the preparation of NiCo2O4 and NiCo2O4/C composites.

20

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(400)

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Intensity (a.u.)

(311)

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30

40

50

60

NCO/C-3

NCO/C-2 NCO/C-1 NCO-350

70

80

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2θ (degree)

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Fig. 1. XRD patterns of NiCo2O4/C with different content of filter paper derived carbon (0, 0.1, 0.2 and 0.3 g).

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(b)

O 1s C 1s

NiCo2O4/C

Intensity (a.u.)

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Co 2p1

NiCo2O4/C

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Ni 2p3

Intensity (a.u.)

(a)

NiCo2O4

1000

800

600

400

200

NiCo2O4

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NiCo2O4/C

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Intensity (a.u.)

(c)

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Binding energy (eV)

0

815 810 805 800 795 790 785 780 775 770

Binding energy (eV)

885

880

875

870

865

860

855

850

Binding energy (eV)

(d)

Intensity (a.u.)

1200

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NiCo2O4

NiCo2O4/C

NiCo2O4

536

534

532

530

528

526

Binding energy (eV)

Fig. 2. XPS spectra for NiCo2O4 and NiCo2O4/C composites: (a) survey spectra (b) Ni 2p spectra, (c) Co 2p spectra and (d) O 1s.

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(a)

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(b) 5 μm

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5 μm

50 μm

20 μm

(c)

20 μm

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(d)

500 nm

20 μm

500 nm

Fig. 3. SEM images of (a) NiCo2O4 and NiCo2O4/C composites with different content of filter carbon (b) NCO/C-1, (c) NCO/C-2 (inset: the corresponding EDS spectrum) and (d) NCO/C-3 at different magnifications.

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(a)

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(b)

1 μm

100 nm

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(c)

(d)

200 nm

C

Ni

Co

O

500 nm

Fig. 4. TEM images of (a) filter carbon, (b) NiCo2O4 (c) NiCo2O4/C composites and (d) elemental mapping images of an individual fiber of NiCo2O4/C composites.

100 50 0

-100

0.0

0.1

0.2

0.3

0.4

0.5

200 150 100

EP

50

AC C

0 -50 -100 -150 -0.1

0.0

0.1

100 50 0 -50 -100

(d)

-1

5 mV s -1 10 mV s -1 20 mV s -1 30 mV s

0.2

0.3

Potential (V)

0.4

0.5

-1

5 mV s -1 10 mV s -1 20 mV s -1 30 mV s

-150 -0.1

0.6

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(c) Current density (A g-1)

Potential (V)

150

Current density (A g-1)

-150 -0.1

200

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-50

(b)

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150

-1

5 mV s -1 10 mV s -1 20 mV s -1 30 mV s

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200

Current density (A g-1)

(a) Current density (A g-1)

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0.6

0.0

200 150 100

0.1

0.2

0.3

0.4

0.5

0.6

0.4

0.5

0.6

Potential (V) -1

5 mV s -1 10 mV s -1 20 mV s -1 30 mV s

50 0 -50 -100 -150 -0.1

0.0

0.1

0.2

0.3

Potential (V)

Fig. 5. Cyclic voltammetry curves of (a) pure NiCo2O4 (NCO-350) and NiCo2O4/C composites: (b) NCO/C-1, (c) NCO/C-2 and (d) NCO/C-3 at various scan rates in 6 M KOH aqueous solution.

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(a)

(b)

-1

1Ag -1 2Ag -1 5Ag -1 10 A g

0.4

Potential (V)

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0.2

0.1

0.0

0.0

400

(d)

0.1

0.0 300

400

Times (s)

500

600

700

400

500

600

700

Times (s)

1500 1200 900 600 300 0

2

4

6

8

Current density (A g-1)

0

(f)

NCO-350 NCO/C-1 NCO/C-2 NCO/C-3

AC C

200

300

0.1

10

200

400

600

800

1000

1200

1400

Times (s) 700

Capacitance retention (%)

0.2

100

200

1800

0.3

0

100

(e)

-1

1Ag -1 2Ag -1 5Ag -1 10 A g

0.4

0

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300

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200

Times (s)

EP

100

Specific capacitance (F g-1)

0

0.2

0.0

Specific capacitance (F g-1)

0.1

0.3

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0.2

-1

1Ag -1 2Ag -1 5Ag -1 10 A g

0.4

0.3

Potential (V)

Potential (V)

0.3

Potential (V)

(c)

-1

1Ag -1 2Ag -1 5Ag -1 10 A g

0.4

600

100

500

80

400

60

300 40 200 20

100 0 0

500

1000

1500

2000

2500

Cycle number

Fig. 6. The galvanostatic charge-discharge curves of (a) NCO-350, (b) NCO/C-1, (c) NCO/C-2, (d) NCO/C-3 at various current densities, (e) Specific capacitance calculated from GCD curves versus current densities and (f) cycling performance of NCO/C-2 during 3000 cycles at scan rate of 100 mV s-1.

0 3000

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(b)

Active material Activated carbon

40 20 0 -20 -40 -60 -80 -1.0

-10

-20 0.0

0.2

0.4

0.6

AC C

EP

0

0.8

1.0

Potential (V)

1.2

-0.8

-0.6

-0.4

-0.2

0.0

0.2

0.4

0.6

Potential (V)

15 12

9

Current (A)

10

(d)

0-0.8 V 0-1.0 V 0-1.2 V 0-1.4 V 0-1.5 V 0-1.6 V

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20

-1

Current density (A g )

(c)

Activated carbon NCO/C-2

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Separator

60

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6 M KOH

Current collector

80

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-

load

Current density (A g-1)

+

(a)

6

-1

20 mV s -1 40 mV s -1 60 mV s -1 80 mV s -1 100 mV s

3 0 -3 -6

1.4

1.6

-9 0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

Potential (V)

Fig.7. (a) Schematic of the assembled NCO/C-2//AC asymmetric supercapacitor and cyclic voltammetry curves of (b) activated carbon and NCO/C-2 in 6 M KOH solution at a scan rate of 20 mV s-1, (c) NCO/C-2//AC device with different potential window at a scan rate of 100 mV s-1, (d) NCO/C-2//AC at various scan rates.

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0.4

0.0 0

200

300

400

Times (s)

500

(d)

22

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20 18

EP

16

12 10 0

2

AC C

14

4

6

Power density (kW kg-1)

60

50

40

8

30

0

2

4

6

8

10

Current density (A g-1)

70

83.04% retention

60

100 90 80

50 70 40

60 50

30 40 20 0

1000

2000

3000

4000

30 5000

Capacitance retention (%)

Energy density (Wh kg-1)

100

70

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0.8

Specific capacitance (F g-1)

Potential (V)

1.2

(c)

Specific capacitance (F g-1)

-1

0.5 A g -1 1Ag -1 2Ag -1 5Ag -1 10 A g

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(b)

1.6

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(a)

Cycle number

Fig. 8. Electrochemical performances of NCO/C-2//AC ASC device: (a) the galvanostatic chargedischarge curves at various current densities; (b) Specific capacitances calculated from GCD curves; (c) Ragone plot; (d) Cycling performance during 5000 cycles at a scan rate of 100 mV s-1.

ACCEPTED MANUSCRIPT Highlights 1. Filter paper derived carbon (FC)/NiCo2O4 was prepared by hydrothermal method. 2. The FC with open channels increased the electron and ion transport rates.

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3. The FC-based supercapcitor exhibited good capacity and cycling stability.