Nanowires for High-Performance Supercap - American Chemical

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Preparation of hierarchical spinel NiCo2O4 nanowires for high-performance supercapacitors Chen Hao, Saisai Zhou, Junjie Wang, Xiaohong Wang, Haiwen Gao, and Cunwang Ge Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.7b04412 • Publication Date (Web): 30 Jan 2018 Downloaded from http://pubs.acs.org on January 30, 2018

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Industrial & Engineering Chemistry Research

Preparation of hierarchical spinel NiCo2O4 nanowires for high-performance supercapacitors a,*

a

a

a

Chen Hao , Saisai Zhou , Junjie Wang, Xiaohong Wang *，Haiwen Gao a b

a

b

and Cunwang Ge

School of Chemistry and Chemical Engineering, Jiangsu University, Zhenjiang, Jiangsu 212013, China School of Chemistry and Chemical Engineering, Nantong University, Nantong, Jiangsu 226019, China

Abstract: Hierarchical spinel NiCo2O4 nanowires were synthesized by a facile hydrothermal method followed by an annealing treatment. The prepared NiCo2O4 nanowires presented a hierarchical mesoporous structure. Moreover, the effects of precipitant and solvent on morphologies of NiCo2O4 were researched. The results reveal that the hierarchical mesoporous structured NiCo2O4 exhibited corking supercapacitor performance with a high specific capacitance of 2876 F·g-1 at a current density of 1 A·g-1，even increased to 10.0 A·g-1, the specific capacitance could still remain 1290 F·g-1, what’s more, the capacitance retention reached 84.7% after 500 cycles, which indicates an excellent electrochemical performance. Those results demonstrate hierarchical mesoporous NiCo2O4 potential to be a promising supercapacitor electrode materials and inspire furthered research on binary metal oxides as charge storage materials and offer a new route for the large-scale production of high-performance supercapacitor electrode materials. Keywords: NiCo2O4; hierarchical; mesoporous; supercapacitor; hydrothermal method.

Introduction Nowadays, the energy crisis and environmental pollution have caught more and more

∗ Corresponding author. Tel.: +86 511 88791800; fax: +86 511 88791800.

E-mail addresses: [email protected] (C. Hao); [email protected] (X.H. Wang)

ACS Paragon Plus Environment

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attention, so there is a growing demand for friendly alternative energy sources and high efficiency energy storage devices with high power density, long lifespan and high reliability. 1 -3 Many efforts have been made in developing high-performance energy storage devices.4-5 However, traditional lithium-ion batteries suffer from a somewhat slow power delivery or uptake. Supercapacitors as a charge storage device have attracted tremendous interest in view of their great advantages, such as large specific capacitance, rapid charge-discharge rate, fast energy delivery, high power performance, long cycle life and environment-friendliness.6-7 Basically, supercapacitors provide large specific capacitance through the ion transport in electrolyte solution and the reversible redox reaction between the active material and electrolyte ion.8-10 In recent years, a variety of electrode materials including carbon materials, conductive polymer and metal oxides, have been extensively studied. Especially, some of transition metal oxides and hydroxides, such as RuO2, MnO2, NiO, Co3O4, Co(OH)2, Ni(OH)2 and so on, have been subject to much more attention because of their high theoretical specific capacitance. In addition, these materials are usually used to fabricate potential electrodes for supercapacitors.11-15 In particular, nickel oxide (NiO) is considered as a promising anode material for supercapacitors owing to its strong theoretical capacity, high energy density, well-maintained fascinating morphology, suitable pore size and large specific surface area and low cost.16-18 However, because of the high resistivity, low rate behavior and poor cycle stability, the capacitive performance of NiO (p-type semiconductor) has been extremely restricted.19-21 Therefore, during the charge/discharge process, the undesirable electrical conductivity hinders its practical applications as electrode materials. Further efforts have been made to prepare the nanocomposites that combine NiO with other electroactive materials.22 Among various electroactive materials, Cobalt oxides (Co3O4 and CoO) have been attached more importance owing to their high theoretical specific capacitance, low cost, good corrosion stability and environment-friendly


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nature.23-24 In addition, -ternary metal oxides show a superior capacitive performance than single component metal oxides due to their richer redox active sites and better electronic conductivity resulting from the synergistic effect of the binary or ternary metal ions.25-27 Ternary nickel cobaltite (NiCo2O4) has recently received more intense attention, because ternary NiCo2O4 possesses better electrochemical activity, higher electrical conductivity and lower electron transport activation energy than binary nickel oxide (NiO) and cobalt oxide (Co3O4). Ternary NiCo2O4 exhibits not only large power density, but also high energy density.28-30 Works of different researchers demonstrated that ternary NiCo2O4 has been regarded as the desirable electrode materials for supercapacitors due to the contributions from the cobalt and nickel ions with different valence states.31-32 In addition, there are so many good features of NiCo2O4, such as high capability, good redox activity, low cost, natural abundance and non-toxic.33-35 The performance of NiCo2O4 could be improved via synthesizing proper nanostructures with high surface area and fast electron/ion transport pathways, so plenty of methods have been made to synthesize NiCo2O4 with various nanostructures including nanoparticles, nanoplatelets, nanosheets, nanowires, nanotubes, microspheres and nanoflowers.36 Nevertheless, as is described in most literatures, the NiCo2O4-based anode materials suffer from poor capacity retention rate resulting from the drastic volume expansion and contraction during the charge/discharge process.37 Although the electrochemical performance of NiCo2O4 has been enhanced through improving the structure and morphology in the previous reports, there are many problems to be solved,38-39 such as low capacity, high charge-discharge overpotential and their unfavorable crystal structure.40 Above all, compared with the two-dimensional (2D) and three-dimensional (3D) structure, 1D NiCo2O4 nanomaterials are expected to have more excellent performance for the reasons that they can reduce the ion transport pathways of faradaic reactions and promote electronic/ionic conductivity, and that they can generate more interfacial active sites


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along the long dimensions and the short dimensions to promote the electron transport.41-42 Moreover, after calcination or annealing process, a large amount of mesopore structure was gained with the hydroxide precursor being converted to NiCo2O4, and the characteristics of mesoporous materials not only ensure the efficient contact between the active material and the electrolyte, but also provide space for volume expansion in the cycles of discharge and charge processes. On the other hand, hierarchical spine NiCo2O4 also can be deposited on flexible substrate to improve the electrochemical performance of flexible supercapacitors, these flexible substrate could be biomass porous substrates like some carbon materials,43 which richly exploit the potential of individual materials, such as short diffusion path, superior current collection and rich electroactive site. For example, Zhang et al.44 reported hierarchical NiCo2O4@NiCo2O4 core/shell nanostructure on flexible cotton activated carbon textiles. In this work, a facile hydrothermal synthesis method followed by a thermal treatment was used to prepare hierarchical mesoporous NiCo2O4 nanowires, which can yield products with high surface area, small crystallite size and high conductivity. The effects of precipitant and solvent on the morphologies of NiCo2O4 were studied and the obtained NiCo2O4 products with different morphologies finally resulted in the diversity in their performance. The simple and effective approach developed an electrode material with stable and favourable performance by integrating transition metal oxides to improve the performance of NiCo2O4 nanowires. Furthermore, the hierarchical nano-sized structure and the features of mesoporous materials are of benefit for the improvement of electrochemical capacity. Moreover, the results of electrochemical measurements showed that the specific capacitance can reach 2876 F·g-1 at a current density of 1 A·g-1 and the NiCo2O4 nanowires exhibited excellent cycle stability with 84.7% capacitance retention after 500 charge-discharge cycles. This value compares favorably to the previous reported other metal oxide electrode materials such as MnO2/ZTO/CMF hybrid composite (642.3 F·g-1 at 1 A·g-1),45 NiCo2O4@NiCo2O4/ACT(1929F·g-1at 1 A·g-1),44 MnO2/polypyrrole


4

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nanorod composite(294 F·g-1at 1 A·g-1).46 This work not only proposes a simple route for improving the capacitance of NiCo2O4, but also provides a guideline for developing green and ideal electrode materials for supercapacitors.

Experiment Material preparation

Scheme 1. Schematic of the preparation process of NiCo2O4 nanowires. As shown in Scheme 1, NiCo2O4 nanowires (NiCo2O4-N) were prepared by a facile hydrothermal synthesis method. Firstly, 0.4351 g of nickel nitrate hexahydrate (Ni(NO3)2·6H2O) and 0.8739 g of cobalt nitrate hexahydrate (Co(NO3)2·6H2O) were dissolved in 40 ml deionized water and ethylene glycol (V distilled water: V ethylene glycol =1:1) under continuous magnetic stirring. Then 0.8106 g urea was added into the above solution and was stirred for 0.5 h to form a uniform mixture. Subsequently, the solution was transferred into a 50 ml Teflon-lined autoclave which was maintained at 160°C for 12 h. After the autoclave was cooled to room temperature naturally, the product was collected by centrifugation, washed with deionized water and absolute ethanol for three times respectively and dried at 60°C for 12 h in an oven. Whereafter, the obtained precursor was calcined at 350 °C for 2 h in air to get the NiCo2O4-N. For comparison, NiCo2O4 microspheres (NiCo2O4-S) were hydrothermally prepared


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under the same conditions except that the solvent was deionized water without ethylene glycol. Similarly, NiCo2O4 nanoplatelets (NiCo2O4-P) were synthesized using the same method in addition that urea was replaced by 1.252 g hexamethylene tetramine (HMT) as the precipitant. Material characterization The phase structures of the as-synthesized products were investigated by the powder X-ray diffraction (XRD, Bruker D8 ADVANCE) using Cu Kα radiation (λ=1.5406 Å) at a scanning rate of 7° min-1 with 2θ ranging from 10° to 80°. The morphology and size of the products were examined by transmission electron microscopy (TEM, JEM-2100) and scanning electron microscopy (SEM, JSM-6480). The Brunauer-Emmett-Teller (BET) surface areas of the samples were determined using a Tristar-3000 surface area analyzer relying on N2 adsorption-desorption. The pore size distribution and pore volume were acquired from the analysis of the desorption branch of the isotherm according to Barrett-Joyner-Halenda

(BJH)

algorithm.

Fourier

transform

infrared

(FT-IR)

spectroscopy was performed on a Nicolet Nexus 470 spectrometer using pure KBr as the background. Electrochemical measurements The electrochemical properties of as-prepared materials were carried out on a CHI660C electrochemical workstation (Chen Hua Instruments, Shanghai, China) with 6 M KOH aqueous as the electrolyte in a three-electrode system. Pt foil and Hg/Hg2Cl2 (Saturated KCl) were used as the counter and reference electrodes, respectively. The working electrodes were prepared as follows: 80wt % of as-prepared samples, 10wt % of acetylene black and 10wt % polytetrafluoroethylene (PTFE) binder in ethyl alcohol solvent were mixed to obtain a homogeneous slurry. The slurry was then coated on a nickel foam (1 × 4 cm2) substrate and dried in a vacuum at 60 °C for 12 h. Subsequently, the nickel foam with the slurry was pressed under a pressure of 10 MPa. The


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electrochemical properties of the as-prepared materials were characterized by cyclic voltammetry (CV) in a potential window between 0 and 0.5V, galvanostatic charge-discharge (GCD) in a potential window between -0.15 and 0.35V and the electrochemical impedance spectroscopy (EIS) techniques with a frequency range from 0.01 Hz to 100kHz. The specific capacitance of the electrode materials can be calculated depending on the charge-discharge curves according to the following equation: CS =

C I∆t = m ∆Vm

(1)

where Cs (F·g-1 ), I (mA), △t (s), △V (V) and m (mg) are the specific capacitance, the charge-discharge current, the discharge time, the potential window of discharge process and the mass of the active materials in the electrodes, respectively.47

Results and discussion Structural and morphological characterization

NiCo2O4-N NiCo2O4-S NiCo2O4-P

10

20

30

40

50

(440)

(422) (511)

(311) (400)

(111)

(220)

Intensity(a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


60

70

80

2θ(degree)

Figure 1. XRD patterns of the as-prepared NiCo2O4-N, NiCo2O4-S, NiCo2O4-P.


7


Figure 1 shows the XRD patterns of NiCo2O4-N, NiCo2O4-S, NiCo2O4-P, and the corresponding results demonstrate that the NiCo2O4 nanocomposites have the same spinel crystal structures. The identified diffraction peaks in the XRD patterns can be readily indexed as a pure cubic phase of NiCo2O4 (JCPDS card No. 20-0781). No other additional diffraction peaks or other phases are observed, indicating that NiCo2O4 nanomaterials with high purity are obtained after a simple annealing process. It is worth noting that the peaks of NiCo2O4-N are broadened with decreased intensity compared with the NiCo2O4-S, NiCo2O4-P, thus indicating a decrease in the grain size. 48-49 This is consistent with the results of the SEM.

a

2p2/3

Co

2+

2p3/2

Co

3+

Co

3+

Ni Intensity (a.u.)

Co 2+

Sat.

810

Ni 2p

b

Co 2p 2p1/2

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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805

800

795

790

785

780

775

770

2p1/2

885

Binding Energy (ev)

880

Sat. Ni

Sat.

875

3+

870

Ni

3+

865

860

855

2+

850

Binding Energy (ev)

Figure 2. High-resolution XPS spectra of the as-prepared NiCo2O4-N nanowires: (a) Co 2p and (b) Ni 2p. The more detailed elemental composition and metal oxidation states of the NiCo2O4-N nanowires were further characterized by X-ray photoelectron (XPS) measurements and the corresponding results are presented in Figure 2a and b. By applying a Gaussian fitting method, the Co 2p spectrum (Figure 2a) could be well-fitted with two spin-orbit doublets and one shake-up satellite (denoted as “Sat.”). Specifically, the fitting peaks at the binding energies of 778.9 and 794.4 eV are attributed to Co3+, while the fitting peaks sitting at 779.7 and 795.7 eV are ascribed to Co2+.36 Similar to Co 2p, the Ni 2p spectrum (Figure 2b) was also fitted with two spin-orbit doublets and two shake-up satellites in the


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same way. The result exhibits that the binding energies at 853.4 and 871.7 eV are assigned to Ni2+, while at 854.8 and 873.8 eV belong to Ni3+.50-51 These above data clearly demonstrate that the component in the surface of the as-prepared NiCo2O4-N nanowires contains Co2+, Co3+, Ni2+ and Ni3+, which may provide enough active sites for the redox reaction of electrode materials. Moreover, from FTIR spectra of the samples, the peaks at around 419 cm-1 are ascribed to the metal-oxygen vibrations of the as-prepared NiCo2O4-N, NiCo2O4-S, NiCo2O4-P (Figure 3).

250

200

Transmittance (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


150

100

419 NiCo2O4-P NiCo2O4-S

50

NiCo2O4-N 500

1000

1500

2000

-1

Wavenumber (cm )

Figure 3. FT-IR spectra of the as-prepared NiCo2O4-N, NiCo2O4-S, NiCo2O4-P.


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Figure

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4. SEM images of NiCo2O4-N (a and b), NiCo2O4-S (c and d) and NiCo2O4-P

(e and f) at different magnifications. The specific structural and morphological of the as-prepared samples are shown in Figure 4. Figure 4a and b illustrate the SEM images of the NiCo2O4-N, displaying that the high density NiCo2O4-N nanowires were uniformly distributed after a simple annealing process. The entire nanowires have a length extended to several hundred


10

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nanometers with an average diameter of approximately 20 nm under SEM observation (Figure 4b). Such small crystal sizes are bound to make the NiCo2O4-N nanowires possess a higher specific surface area that they may contact with the electrolyte ions more effectively, which is favorable for the full utilization of the active materials. Thus, it is expected that NiCo2O4-N nanowires have satisfactory specific capacitance. From Figure 4c, it is clearly observed that NiCo2O4-S exhibits an obvious 3D micro-spheres structure with the diameter of 9 um averagely. The high-magnification SEM image (Figure 4d) exhibits that the 3D NiCo2O4-S, with the appearance like a dandelion, is actually constructed with radial ultrafine nanowires. The morphology of the NiCo2O4-P was also observed by SEM (Figure 4e and f), showing the irregular multilayer nanoplatelets formation of 700-800 nm in diameter, and this morphology of the NiCo2O4 could enhance the flexibility during the charge-discharge process. When the precipitant or solvent system was changed, the NiCo2O4 with different morphologies could be prepared by a facile hydrothermal synthesis method.

Figure 5.

TEM images of sample NiCo2O4-N (a and b) at different magnifications.

As can be seen from Figure 5a and b the individual NiCo2O4 nanowire has a mesporous structure and well crystallized with clearly resolved hierarchical NiCo2O4 (Figure 5b). The TEM images demonstrate the same hierarchical mesporous structure, these highly porous structures are composed of nanocrystallites approximately 20 nm in


11


size. This unique mesoporous morphological characteristic is expected to allow an easy penetration of the electrolyte into the NiCo2O4 nanowires and effective accommodation of the volume expansion during the charge-discharge process, thereby leading to enhanced electrochemical reactivity.

0.018

NiCo2O4-N

b

NiCo2O4-S

c

NiCo2O4-P

a

0.016 0.014

NiCo2O4-N

b

NiCo2O4-S

c

NiCo2O4-P

-1

a

dV/ dD (cm g nm )

160

0.012

-1

120 a

80

3

Volume Adsorbed (cm3/g)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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b c

40

a

0.010 0.008

c 0.006

b 0.004

0 0.0

0.002

0.2

0.4

0.6

0.8

1.0

0

2

4

6

8

10

12

14

16

Pore Size (nm)

Relative Pressure (p/p0)

Figure 6. N2 adsorption-desorption isotherms of NiCo2O4-N, NiCo2O4-S and NiCo2O4-P. The texture properties were further elucidated by N2 adsorption/desorption isotherms. Figure 6a shows the resulting N2 adsorption-desorption isotherms of the NiCo2O4-N, NiCo2O4-S and NiCo2O4-P, the BET surface area of NiCo2O4-N, NiCo2O4-S and NiCo2O4-P were 69.4, 66.1 and 66.9m2·g−1, respectively. There are three distinct hysteresis loops can be observed in the range of 0.6-1.0 P/P0, which suggest the presence of a mesoporous structure for the NiCo2O4-N, NiCo2O4-S and NiCo2O4-P,52-53 being in good agreement with the above TEM observations. As shown in the Figure 6b, the pore size distribution of the samples were calculated using desorption isotherm by Barret-Joyner-Halenda (BJH) method. The NiCo2O4-N, NiCo2O4-S and NiCo2O4-P show average pore size at 6.8 nm, 5.1 nm and 5.8 nm, and the samples pore volume of the NiCo2O4-N, NiCo2O4-S and NiCo2O4-P were 0.14 cm³/g, 0.093 cm³/g, 0.12 cm³/g, respectively. The results show that the NiCo2O4-N have a large specific surface area and pore size compared with NiCo2O4-S and NiCo2O4-P, These will be have a more effective contact between active materials and the electrolyte. which is beneficial to the electrochemical performance.


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Electrochemical Behaviors of the Electrodes.

75

a

b

50

20 10 0 -10

NiCo2O4-N NiCo2O4-S

-20

NiCo2O4-P

Current Density(A g-1)

30

Current Density(A g-1)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


25 0 -1

5mv s -1 10mv s -1 15mv s -1 20mv s -1 25mv s -1 50mv s

-25 -50

-30

-75 0.0

0.1

0.2

0.3

0.4

0.5

Potential(V vs.SCE)

0.0

0.1

0.2

0.3

0.4

0.5

Potential(V vs.SCE)

Figure 7. (a) CV curves of the NiCo2O4-N, NiCo2O4-S and NiCo2O4-P composites at 5 mv·s-1; (b) CV curves of NiCo2O4-N at different scan rates in 6 M KOH solution. The electrochemical performance of as-prepared NiCo2O4-N, NiCo2O4-S and NiCo2O4-P as the electrodes material for supercapacitors is evaluated by using 6 M KOH aqueous solution as the electrolyte. As demonstrated in Figure 7a, compared the CV curves of the NiCo2O4-N, NiCo2O4-S and NiCo2O4-P electrodes at a scan rate of mv s-1, the area surrounded by the CV curve of NiCo2O4-N electrode is evidently larger and the redox current density is higher than the CV curves of NiCo2O4-S and NiCo2O4-P electrodes, indicating the NiCo2O4-N electrode have the much more superior capacitive behavior. Additionally, with the increase of scan rate (from 5 to 50 mv·s-1) in Figure 7b, the redox peaks locate at around 0.388 and 0.046 V at a slow scan rate of 5 mv·s-1, which is mainly attributed to the Faradaic redox reactions related to M-O/M-O-OH, where M refers to Ni or Co. The shapes of these CV curves of NiCo2O4-N electrode just slight shape distortion are observed except when the scan rate is 5 mv·s-1, due to the increase of scan rate leading to the existence of polarization behavior, indicating high rate capacity of the as-prepared NiCo2O4-N, this result is in good agreement with GCD analysis ( Figure


13


7b). 0.4

0.4

a NiCo2O4-N NiCo2O4-P

0.2

0.5Ag -1 1Ag -1 2Ag -1 3Ag -1 5Ag -1 10Ag

0.3

NiCo2O4-S Potential(V vs.SCE)

Potential(V vs.SCE)

0.3

-1

b

0.1 0.0

0.2 0.1 0.0 -0.1

-0.1 0

500

1000

1500

2000

2500

3000

0

3500

1000 2000 3000 4000 5000 6000 7000 8000

Time(s)

Time(s) 1000

c

3500

NiCo2O4-N Specific Capatiance(F g-1)

NiCo2O4-S

-1

Specific capacitance (F g )

3000

NiCo2O4-P

2500 2000 1500 1000

100

d

800

80

600

60

400

40

200

20

500 0

0 0

2

4

6

8

0

10

e

200

300

400

0 500

Cycle Number

-1

Current density (A g )

100

100

NiCo2O4-N NiCo2O4-S

80

NiCo2O4-R 60 40 -Z''(Ohm)

-Z''(Ohm)

10

20

5

0

0

1

2

3

4

5

Z'(Ohm)

0

20

40

60

80

100

Z'(Ohm)

Figure 8 (a) galvanostatic charge-discharge curves of the three electrode materials at a current density of 1 A·g-1, (b) galvanostatic charge-discharge curves at various current densities ranging from 0.5 to 10 A·g-1, (c) the specific capacitance of the three electrode materials at various current densities, (d) the cycling performance of NiCo2O4-N at a current density of 10 A·g-1, (e) Nyquist plots of NiCo2O4-N, NiCo2O4-S and NiCo2O4-P


14

Retention(%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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electrode materials. The inset is the enlarged Nyquist plots at the high frequency region. Table1 .Comparison of the as prepared NiCo2O4-N nanowires electrode material with previously published results Reference

Electrode

Voltage(V)

This work

NiCo2O4-N

46

MnO2/polypyrrole nanorod MnO2/ZTO/CMF

45 44

0.4

Specific Capacitance(F g-1) 2876 (1 A g-1)

6 M KOH

1

294 (1 A g-1)

1 M Na2SO4

0.8

642.3 (1 A g-1)

1 M Na2SO4

-1

Electrolyte

0.35

1929 (1 A g )

6 M KOH

1

NiCo2O4@NiCo2O4/A CT Ni@NiCo2O4

0.5

899 (1 A g-1)

6 M KOH

2

NiCo2O4/CC

1

1768 (5 mA cm-2)

6 M KOH

To further evaluate the application potential of the as-prepared NiCo2O4-N, NiCo2O4-S and

NiCo2O4-P

electrodes

for

supercapacitors,

galvanostatic

charge-discharge

measurements to evaluate the specific capacitance of the electrode were carried out in a 6 M KOH electrolyte between 0.15 and 0.35 V (vs. SCE), which is mainly originates from the Faradaic redox reactions related to M-O/M-O-OH, where M refers to Ni or Co.54 The specific capacitances are superior to most pioneering NiCo2O4-based electrodes with different morphologies and compositions. Figure 8a presents the galvanostatic charge-discharge (GCD) curves of the three electrodes at a current density of 1 A·g-1. According to the equation (1), it can be calculated that the specific capacitance of NiCo2O4-N, NiCo2O4-S and NiCo2O4-P are about 2876 F·g-1, 1344 F·g-1, 950 F·g-1, while that of NiCo2O4-N nanowires is about 1290 F·g-1 at a current density of 10 A·g-1 as show in Figure 8b. This result further demonstrates that the NiCo2O4-N nanowires has much better electrochemical performance than that of the 3D NiCo2O4-S and the NiCo2O4-P multilayer nanoplatelets. There are two visible voltage plateaus in the charge and discharge curves. Remarkably, it can be seen from Figure 8a that the discharge curves of the as-prepared NiCo2O4-N, NiCo2O4-S and NiCo2O4-P electrodes consist of two sections: a rapid potential drop due to the internal resistance and a sluggish potential decay because


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of the Faradaic redox reaction. The calculated specific capacitances of the as-prepared NiCo2O4-N, NiCo2O4-S and NiCo2O4-P electrodes at different current densities ranging from 0.5 to 10 A·g-1 are plotted in Figure 8c, it can be seen that the NiCo2O4-N nanowires exhibit excellent specific capacitances of 3400 F·g-1, 2876 F·g-1, 2268 F·g-1, 1968 F·g-1, 1698 F·g-1, 1290 F·g-1 at current densities of 0.5, 1, 2, 3, 5 and 10 A·g-1, respectively. The specific capacitances of the NiCo2O4-S (NiCo2O4-P) electrode are calculated to be 2363(1306), 1344(950), 973.6(714.4), 840(605.4), 697(520) and 452(386) F g−1 at current densities of 0.5,1, 2, 3, 5 and 10 A·g-1, respectively. This suggests that about 44.8% of the specific capacitances at 1 A g−1 is still retained when the discharge current density is increased from 1 to 10 A·g-1, which is much higher than the specific capacitances of NiCo2O4-S and NiCo2O4-P, the specific capacitances of NiCo2O4-S and NiCo2O4-P electrodes are only 452 F g−1 and 386 F g−1at 10 A g−1, corresponding to around 33.6% and 40.6 % retention at 1 A g−1. These results demonstrate that the NiCo2O4-N is preferable to improve the rate performance. The loss of the capacitance at high current densities is attributed mainly to the huge volumetric expansion associated with repeated multielectron Faradic reactions.55 The performance of NiCo2O4-N electrode is remarkable compared with NiCo2O4-S and NiCo2O4-P electrodes, the NiCo2O4-N electrode exhibits significantly enhanced specific capacitance, the high capacitance is connected with the area of substrate or mass of the active material.56 This improved electrochemical performance could be related to the advantageous structural features of these NiCo2O4-N. Specifically, the small diameter of the nanowires with high porosity leads to a high surface area of about 66.1 m2 g−1, and thus the electrolyte can easily penetrate through the for efficient redox reactions during the Faradaic charge storage process. Second, the separation of neighbouring nanowires from each other and makes most of the surface of nanowires easily accessible by the electrolyte, which will give high capacitance and enhanced electrochemical kinetics relative to conventional film electrodes.


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In addition, a good cycling stability is an essential requirement for any kind of energy storage device and a long cycle life for large scale application. The long-term cycling performances of the NiCo2O4-N electrode are subjected to continuous galvanostatic charge-discharge measurement at a current density of 10 A·g-1 and the special capacitances retention of the NiCo2O4-N device as a function of charge-discharge cycles is shown in Figure 8d, the special capacitances of the NiCo2O4-N electrode gradually decreases and then remains unchanged during the 500 cycles, the cycling is very stable with a specific capacitance of 800 F·g-1 retained over 500 cycles, corresponding to a loss of only 15.2% relative to the first cycle (calculated from the discharge-charge capacity), which can be attributed to the full activation of the crystalline NiCo2O4 nanowires electrode. Before this measurement, the specific capacitance is 1290 F·g-1 at current densities of 10 A·g-1, this capacity decline behavior might be caused by the loss of active material during the charge-discharge cycles. The outer surface of the electrode materials is available only for electrolyte access, leaving the dead region in the bulk position due to the limited diffusion of electrolyte ions at high current density. Based on the above overall electrochemical performance, the unique ultrathin mesoporous NiCo2O4 nanowires have the high-performance supercapacitors characterized by both long cycle life and excellent rate capability. It is well known that the electrochemical performance is largely related to the ion diffusion and charge transfer processes. To understand the electrochemical characteristics of NiCo2O4-N, NiCo2O4-S and NiCo2O4-P, electrochemical impedance spectroscopy (EIS) measurement was carried out. The Nyquist plots of the electrodes are shown in Figure 8 e at the open circuit potential in the frequency range of 100 kHz-0.1 Hz. In the high frequency region, the intersection of the curve on the real axis indicates the resistance of the electrochemical system (Rs), including the ionic resistance of the electrolyte, the intrinsic resistance of the electrode active material, and the contact resistance between the active material and the substrate,57 and the diameter of the semicircle corresponds to the


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charge-transfer resistance (Rct) of the electrodes and electrolyte interface.58-59 All the EIS spectra are consisted of one semicircles in the high frequency region and a straight line in the low frequency region, which can be simulated by an equivalent circuit. As for the three electrodes, the Rs values are 1.185Ω, 1.727Ω, 1.269Ω, respectively, indicating that the NiCo2O4-N electrode have a lower solution resistance and Faradaic resistance than the NiCo2O4-S and NiCo2O4-P electrodes. In the low frequency range, the almost vertical lines of NiCo2O4-N, NiCo2O4-S and NiCo2O4-P represent the fast ion diffusion in electrolyte and the fast adsorption onto the electrode surface, suggesting the Warburg resistance (described as diffusive impedence of OH ions) is not a determining factor for the ideal capacitive behavior of the electrodes.60 It is obvious that the NiCo2O4 electrode prepared in this work exhibits a high specific capacitance in comparison with that of the reported in the literatures, as demonstrated in Table1.

Conclusions In summary, we have developed a facile hydrothermal method to synthesize hierarchical mesoporous spinel NiCo2O4 nanowires. The solvent was deionized water and ethylene glycol (V

distilled water:

V

ethylene glycol =1:1),

which is the particular feature of this

work. The as-prepared NiCo2O4-N exhibited a high specific capacitance of 2876 F·g-1 at a current density of 1 A·g-1, while the nanowires still maintained a large specific capacitance of 1290 F·g-1 at 10.0 A·g-1 with a capacity retention of 84.7% after 500 cycles. This study shows the function of the hierarchical mesoporous NiCo2O4 nanowires as supercapacitor electrode materials with high activity, large specific capacitance and excellent cycling stability. It can be ascribed to their advantageous structure consisted of hierarchical porous network with the high degree of pore connectivity stemming from the NiCo2O4 nanowires. During the annealing process in air, the formation of the NiCo2O4 structure suffered from the combined effect of contraction and adhesion actions caused by non-equilibrium heat treatment. The comprehensive comparison of these three NiCo2O4 with different morphologies further confirms the significant effect of the crystal structure


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on the electrode materials. A similar design might be applied to many other mixed metal oxides and the hybrid functional materials show a great prospect for the preparation of high performance supercapacitors. This study highlights the importance of a special electrode design and provides a guideline for developing highly efficient electrode materials for supercapacitors.

Acknowledgments We gratefully acknowledge the National Natural Science Foundation of China (61571245), the Natural Science Foundation of Jiangsu Province (BK20131249), the Senior Personnel Scientific Research Foundation of Jiangsu University (15JDG084) and Natural Science Fund Project of Colleges in Jiangsu Province(16KJB430008) for financial support of this research.

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Preparation of hierarchical mesoporous spinel NiCo2O4 nanowires and their electrochemical performance


Nanowires for High-Performance Supercap - American Chemical

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