Three-Dimensional hierarchical structure ZnO@C@NiO on carbon

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Three-Dimensional hierarchical structure ZnO@C@NiO on carbon cloth for asymmetric supercapacitor with enhanced cycle stability Yu Ouyang, Xifeng Xia, Haitao Ye, Liang Wang, Xinyan Jiao, Wu Lei, and Qingli Hao ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b16021 • Publication Date (Web): 03 Jan 2018 Downloaded from http://pubs.acs.org on January 3, 2018

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ACS Applied Materials & Interfaces

Three-Dimensional hierarchical structure ZnO@C@NiO on carbon cloth for asymmetric supercapacitor with enhanced cycle stability Yu Ouyang, Xifeng Xia, Haitao Ye, Liang Wang, Xinyan Jiao, Wu Lei*, Qingli Hao* *

Key Laboratory of Soft Chemistry and Functional Materials, Ministry of Education, Nanjing University of Science and Technology, 210094, China

ABSTRACT: In this work, we synthesized the hierarchical ZnO@C@NiO core-shell nanorods arrays (CSNAs) grown on carbon cloth (CC) conductive substrate by a three-step method involving hydrothermal and chemical bath methods. The morphology and chemical structure of the hybrid nanoarrays were characterized in detail. The combination and formation mechanism was proposed. The conducting carbon layer between ZnO and NiO layers can efficiently enhance the electric conductivity of the integrated electrodes, and also protect the corrosion of ZnO in alkaline solution. Compared with ZnO@NiO nanorods arrays (NAs), the NiO in CC/ZnO@C@NiO electrodes which possess a unique multi-level core-shell nanostructure exhibits higher specific capacity (677 C/g at 1.43 A/g) and enhanced cycling stability (capacity remain 71% after 5000 cycles), on account of the protection of carbon layer derived from glucose. Additionally, a flexible all-solid-state supercapacitor is readily constructed by coating the PVA/KOH gel electrolyte

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between the ZnO@C@NiO CSNAs and commercial graphene. The energy density of this all-solid-state device decrease from 35.7 to 16.0 Wh/Kg as the power density increase from 380.9 to 2704.2 W/Kg with excellent cycling stability (87.5% of the initial capacitance after 10000 cycles). Thereby, the three-dimensional hierarchical structure ZnO@C@NiO CSNAs are promising electrode materials for flexible all-solid-state supercapacitors.

KEYWORDS: supercapacitor, ZnO@C@NiO, hierarchical structure, integration electrode, carbon layer, glucose 1. INTRODUCTION With the vast consumption of fossil-fuel and the rapid depletion of non-renewable resources, it has become essential for the utilization of clean and renewable energy. Among

the

various

energy

storage

devices,

supercapacitors,

also

called

electrochemical capacitors, has received tremendous worldwide concern and increasing research interest due to high power density, fast charge/discharge rates and long cycling duration in comparison to batteries and other conventional storage devices.1-3 Until now, the different types of electrode materials for supercapacitors can be summarized into three major categories: carbonaceous materials, conducting polymers, and transition metal oxides/sulfides or other compounds. However, due to the drawbacks of individual electrode materials, single electrode material may not be able to meet the increasing demand of high energy density and long cycling stability as well as fast charge-discharge rate. For instance, being one of the most used supercapacitors materials, carbonaceous materials have several advantages: low cost, 2


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outstanding cycling performance and remarkable power density; however, the energy density is usually less than 10 Wh/kg. Additionally, conducting polymers and the transition metal oxides/sulfides are widely studied for their good flexibility in controlling structure and morphology, low cost and high capacitance, though, their poor cycling stability limit their practical application in supercapacitors. To resolve the above-mentioned drawbacks, hybrid nanocomposites with different hierarchical structures, morphology are employed.4-7 A series of nano metal oxides, such as Co3O4,8 MnO2,9 NiO,10 NiCo2O4,11 CoMn2O412 and ZnCo2O413, have been studied as new electrode materials for supercapacitors because of their strong electrochemical activities and rich redox reactions. Among the various metal oxides, NiO is considered as one of the most promising materials.14 For instance, compared with another supercapacitor material RuO2, it presents high theoretical specific capacitance (2573 F/g), chemical/ thermal stability, superior redox reactivity, benign environmental impact and economical cost. Nevertheless, the unfavorable kinetics and the low surface area of those metal oxides are common shortcoming and lead to low utilization of active materials. In recent years, great efforts have been devoted to develop higher specific capacitance and structural

stability

by

rationally

designing

the

hybridization

of

different

nanostructured electrode materials. Recently, the core-shell structure based on excellent conductivity substrates as promising solution to improve electrochemical performance, is widely employed in energy storage, such as Co3O4@NiO,15 Co3O4@NiMoO4,16

NiCo2O4@MnO2,17

ZnO@MnO2,18-19

ZnO@Ni(OH)2.20 3



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Especially, zinc oxide (ZnO), as a skeleton for hierarchical structure, is one of the promising semiconductor materials, which possesses high electric conduction properties, chemical and mechanical stabilities.21 For example, Yang et al. developed the flexible electrode of H-ZnO@MnO2 which exhibited high areal capacitance 138.7 mF/cm2 at 1 mA/cm2.22 As reported by Pu et al., Ni(OH)2 on ZnO arrays supported on Ni foam possessed high capacitance (2028 F/g at 10 A/g) and poor cycling properties (68 % of their initial capacitance after 500 cycles).23 Additionally, Xing et al. reported the core-shell Ni3S2@ZnO synthesized by electrodeposited method expressed high specific capacitance (1529 F/g at 2 A/g) and unsatisfied cycle ability (42 % of initial capacitance after 2000 cycles).6 Due to contribution of ZnO in electron transportation and ion diffusion, capacitance of the as-mentioned works above are improved greatly, however, the intrinsic property of facilitating corrosion in alkaline and acid results in the decline of circulation property.

In our work, we develop an integrated core-shell structure of ZnO@C@NiO core-shell nanorods arrays (CSNAs) on carbon cloth (CC) (CC/ZnO@C@NiO) for supercapacitors by a simple and scalable strategy to effectively protect ZnO nanoneedles arrays (ZnO NAs) from corrosion in alkaline solution, and to simultaneously enhance the conductivity of the electrode. It is achieved by fabricating an amorphous carbon layer on ZnO NAs by carbonation with glucoses the precursor, followed by a shell of NiO nanosheets produced using chemical bath approach with calcination at 350 ℃. The designed hierarchical structure of CC/ZnO@C@NiO CSNAs can contribute to the high active surfaces and the enhanced performance of 4


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the electrodes. The NiO in well-designed CC/ZnO@C@NiO electrode exhibits a high specific capacity (677 C/g at 1.43 A/g) and the enhanced cycle ability (capacity remain 71 % after 5000 cycles) compared with the CC/ZnO@NiO electrode. Furthermore,

an

all-solid-state

asymmetric

supercapacitor,

based

on

CC/ZnO@C@NiO//Commercial graphene, delivers the energy density and power density of 35.7 Wh/Kg (at 380.9 W/Kg) and 16.0 Wh/Kg (at 2704.2 W/Kg), respectively, and possesses excellent cycling stability (87.5 % of the initial capacitance after 10000 cycles). The enhanced electrochemical performance of supercapacitors is mainly attributed to the well-ordered ZnO NAs protected with an amorphous carbon layer, and the integrated CC/ZnO@C@NiO with the large specific surface area. As a binder-free electrode for supercapacitors, the unique CC/ZnO@C@NiO electrode also exhibits the good prospect of practical application for supercapacitors.

2. EXPERIMENTAL SECTION 2.1. Chemicals In the current work, all the chemicals were analytical graded and were used without further

purification.

Zinc

nitrate

hexahydrate,

nickel

nitrate

hexahydrate,

hexamethylenetetramine, ammonia, potassium peroxydisulfate and glucose were obtained from Sinopharm Chemical Reagent Co., Ltd. Polyethylene glycol (PVA-1799) was obtained from Aladdin Chemical Reagent Co., Ltd. Commercial graphene was purchased from the Sixth Element (Chang zhou) Materials Technology Co., Ltd. Carbon cloth (WOS 1002) was purchased from Ce Tech Co., Ltd. 5



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2.2. Preparation of ZnO NAs on carbon cloth Before the experiment, the carbon cloth substrate was ultrasonically washed with acetone, water and ethanol for 30 min individually, and then placed in nitric acid under 90 ℃ for 12 h. After that, a seed-assisted hypothermal method was used for fixing ZnO NAs on carbon cloth substrate. The carbon cloth was soaked in 0.005 M zinc acetate then placed in an oven under 300 ℃ for 10 min to form a ZnO seed layer.

And

a

45

mL

precursor

solution

was

prepared

with

0.05

M

hexamethylenetetramine (HMTA) and 0.05 M zinc nitrate hexahydrate and 2 mL ammonia under constant magnetic stirring for 30 min. Later, the prepared carbon cloth substrates were put into the precursor solution then placed under 95 ℃ for 4 h. After the reaction, the white product on carbon cloth was washed with distilled water, and the ZnO NAs on the carbon cloth (CC/ZnO) were obtained.

2.3. Preparation of core-shell ZnO@C nanorods arrays on CC The as-prepared CC/ZnO was dipped in the prepared solution with 1 M glucose in 50 mL deionized water for 24 h. After that, the carbon cloth was subsequently put into tube furnace by carbonization at 800 ℃ in N2 gas for 2 h. The core-shell ZnO@C nanorod arrays on CC substrate were acquired, and the electrode was termed as CC/ZnO@C.

2.4. Preparation of CC/ZnO@C@NiO electrode The CC/ZnO@C was used as the scaffolds for the growth of NiO nanosheets shell via a simple chemical bath method. In a typical procedure, the solution for chemical

6


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bath deposition was prepared by adding 5 mL of ammonia to 45 mL mixture of 0.025 mol nickel nitrate hexahydrate and 0.005 mol potassium peroxydisulfate. The CC/ZnO@C was immersed in the solution for 5 min at room temperature, and then washed with deionized water and ethanol for several times, finally annealed in oven under 350 ℃ for 2 h. The average mass of NiO nanosheets in CC/ZnO@C@NiO was about 0.7 mg/cm2. For comparison, the CC/ZnO@NiO electrode without the inner carbon layer was also prepared by a similar procedure. The CC/NiO was also synthesized by the same method as that for developing NiO nanosheet array shell on the CC/ZnO@C. For comparison, the precursor of NiO nanosheets before calcination was collected from the same breaker. The mass density of NiO in carbon cloth is about 0.7 mg/cm2.

2.5. Fabrication of all-solid-state asymmetric supercapacitor The process of preparing PVA/KOH electrolyte was shown as follows: 6 g PVA was dissolved in 30 mL deionized water while heated in 90 ℃water bath with continually stirring until the transparent solution obtained. In the cooling process, 3.37 g KOH was added to 30 mL deionized water under constantly stirring. The KOH solution was dropped into the cooled gel solution under continually stirring until the clear solution obtained.

The all-solid-device was assembled according to the following method: Firstly, the anode was prepared by mixing the commercial graphene, acetylene black, and polytetrafluoroethylene (1% wt) with the mass ratio 85: 10: 5. After grinding

7



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adequately, the slurry was spread on nickel foam (1×2 cm2), which performed as the anode, while the CC/ZnO@C@NiO electrode (1×2 cm2) was directly used as the cathode. The two electrodes were coated on with a thin layer of PVA/KOH electrolyte and assembled face to face via gently squeezing under room temperature, and then wrapped with PVC film. Later, the device with two plate lugs was tested after sealed for 8 h. Finally, the all-solid-state asymmetric supercapacitor was assembled, termed as CC/ZnO@C@NiO// graphene.

2.6. Characterization of electrode materials Morphology analyses of samples were performed on a Transmission Electron Microscope (TEM, JEOL JEM-2100) and Field Emission Scanning Electron Microscopy (SEM, Quant 250 FEG) with Energy Dispersive Spectrum (EDS). For TEM observation, the samples were first re-dispersed in ethanol by ultrasonic treatment, and then dropped on carbon copper grids. The powder X-ray diffraction pattern (XRD) analyses were carried out on a Bruker D8 Advance diffractometer with Cu Kα radiation (λ=1.5406 Å) whose scanning angle ranging from 10° to 80° of 2θ. Additionally, X-ray photoelectron spectroscopy (XPS) measurements were performed on Thremo Escalab 250 (UK) at monochromatic Al Kα (150 W, 500 µm and 1486.6 eV) radiation.

2.7. Electrochemical Measurement For all electrode materials, the electrochemical measurements were executed with an electrochemical workstation (CHI660C) in a typical three-electrode system with 3

8


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M KOH aqueous solution as the electrolyte. The as-prepared free-standing electrodes were directly tested as the working electrodes. A Pt foil was used as the counter electrode and a Hg/HgO electrode was used as the reference electrode. The electrochemical performance of electrodes and supercapacitors were evaluated with cyclic voltammetry (CV), charge−discharge measurements and electrochemical impedance spectroscopy (EIS) on the electrochemical workstation.

3. Results and discussion 3.1 Synthesis and Characterization

Figure 1. Schematic illustration of the formation of CC/ZnO@C@NiO The fabrication procedure of the highly ordered ZnO@C@NiO CSNAs on carbon cloth is schematically shown in Figure 1. The ZnO nanoneedles were vertically grown on carbon fibers via a seed-assisted hypothermal method and then the substrate was submerged in 1 M glucose solution for 24 h at the room temperature. After that, the 9



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obtained ZnO@glucose was annealed at 800 ℃ under N2 atmosphere in a tube furnace to transform glucose into carbon layer. Then, the NiO nanosheet array shell was prepared by a chemical bath method. After annealed at 350 ℃ in air, the final hierarchical structure of CC/ZnO@C@NiO CSNAs was obtained.

As shown in Figure 2, the morphologies of the CC/ZnO NAs, CC/ZnO@C CSNAs and CC/ZnO@C@ZnO CSNAs were characterized by SEM. Figure 2a and d show the SEM images of the CC/ZnO NAs. It can be clearly seen that the perfectly straight ZnO NAs were grown uniformly on the carbon fiber of CC substrates. A careful examination (Figure 2d and inset) reveals that these needle-like ZnO NAs are 2 µm in length and about 125~250 nm in diameter at the middle section. After immersed in the glucose solution for 24 h and annealed at 800 ℃ in N2, the ZnO NAs were covered with a carbon layer obtained. From the SEM images of CC/ZnO@C (Figure 2b, e), it can be found that the diameter of all nanoneedles becomes much larger and the shape of the ZnO nanoneedles turns into column-like. It is because a thick carbon layer was formed and covered entirely on the surface of all ZnO nanoneedles, moreover, the roots of ZnO NAs were connected by the carbon layer. From the Figure S1, compared to CC/ZnO@C (16.2 m²/g), CC/ZnO@C@NiO delivers higher the specific surface area (39.1 m²/g). So that, such a heterostructure of CC/ZnO@C NAs can be used as a scaffold providing a large specific surface area for further deposition of NiO nanosheets arrays as the shell of a core-shell structure.24

The SEM image (Figure 2c, f) illustrates that the NiO nanosheets homogeneously

10


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cover the whole surface of the CC/ZnO@C nanorods arrays, forming a three-dimensional hierarchical structure. The fluffy structured ZnO@C@NiO CSNAs were well grown on the carbon cloth with widely-distributed pores and channels, which is beneficial for electrolyte to penetrate into the active materials. In this integrated core-shell structure, the inner carbon layer has a better electrical conductivity, high chemical stability and good mechanical property in comparison with ZnO. It can not only protect ZnO NAs from the collapse of the structure in alkaline solution, but also act as an electrical bridge connecting the NiO shell and ZnO core. Therefore, the final core-shell ZnO@C@NiO CSNAs on CC are expected to exhibit the enhanced electrochemical performance compared with CC/ZnO@NiO. The SEM of CC/ZnO@NiO in Figure. S2 also shows the core-shell structure consisted of NiO nanosheet shell and ZnO NAs core, in which the average diameter of the ZnO@NiO nanorodes is approximate 208 nm, thinner than that of ZnO@C@NiO (~280 nm). Moreover, the shell of NiO in ZnO@NiO nanorods is much closer than that in ZnO@C@NiO and presents poor three dimensional skeleton. It might be due to the poor affinity of NiO and ZnO layers.

11



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Figure 2. Typical SEM images of electrodes at different magnifications. (a, d) CC/ZnO; (b, e) CC/ZnO@C; (c, f) CC/ZnO@C@NiO. To further investigate the detailed structural information and morphology of the CC/ZnO@C@NiO

CSNAs,

various integrated

electrodes

were

treated

by

ultrasonication for certain time in order to remove the metal oxides from the carbon clothes. Figure 3a and d show different magnifications of TEM images of the damaged ZnO nanoneedles, which display the perfect straight ZnO nanoneedle with ~200 nm in diameter. A clear lattice spacing of 0.2477 nm can be seen for ZnO (1 0 1) (JCPDS no. 36-1451) from the inset image in Figure 3d. As shown in Figure 3b for ZnO@C, although the sample was destroyed to some degree, it can still be observed that the ZnO nanoneedle is wrapped by carbon layer (~20 nm) in the ZnO@C nanostructures. Further information is presented in high resolution TEM (HRTEM) image (Figure 3e) which shows a spacing plane of 0.26 nm corresponding to the (0 0 2) plane of the wurtzite-type ZnO. And amorphous carbon was produced by the carbonization of glucose under nitrogen. According to the TEM image of ZnO@C@NiO observed in Figure 3c and inset, the multilevel nanostructure was composed of ‘core’ ZnO NAs and ‘shell’ NiO ultrathin nanosheets. Moreover, the 12


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interconnected nanosheets of NiO regularly grow on the surface of carbon layer, leading to a large amount of micropores in the arrays. From the HRTEM of NiO nanosheets around the ZnO@C nanorods (Figure 3f), one can clearly observe that many mesopores with several nanometers remain in the NiO nanosheets due to the thermal treatment of the precursor. The lattice spaces of 0.24 and 0.21 nm in the inset of Figure 3f are corresponding to the (1 1 1) and (2 0 0) planes of NiO (JCPDS no. 65-2901, cubic phase), respectively. The SAED in Figure S3 also verifies that the NiO nanosheets

possess

polycrystalline

structure.

Figure 3. Low-magnification and high-magnification TEM images of (a,d) ZnO nanoneedle, (b,e) ZnO@C, and (c,f) ZnO@C@NiO. The EDS of linear sweep on a ZnO@C@NiO nanorod shows that the elements of Zn, Ni, O, and C (as shown in Figure S4) are detected along the scanning line, which indicate the existence of the above elements on ZnO@C@NiO nanorod. The elemental mapping of CC/ZnO@C@NiO also shows the presence of Zn, Ni, O, and C 13



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elements uniformly dispersed (Figure S5). Furthermore, as we can see from the Figure 4, the elements of C, Ni, O, Zn were dispersed in two ZnO@C@NiO nanorods. The results indicate that the ZnO nanoneedle core, carbon layer, and NiO nanosheet shell are coated on the carbon cloth substrate in good condition.

Figure 4. SEM image (a) and EDS (b) of ZnO@C@NiO; elemental mapping of C (c); Ni (d); O (e); Zn (f). Figure 5 shows the XRD patterns of the carbon cloth, CC/NiO, CC/ZnO and CC/ZnO@C@NiO CSNAs. It is proved that the peak of the carbon cloth is ranging from 26° to 43°. Besides, the weak peaks marked as “♦” belong to (1 1 1), (2 0 0), (2 2 0), (3 1 1) of cubic NiO (JCPDS no. 65-2901). The strong peaks marked as “♥”are shown in the XRD pattern of CC/ZnO@C@NiO, corresponding to (1 0 0), (0 0 2), (1 0 1), (1 0 2), (1 1 0), (1 0 3), (2 0 0), (1 1 2), (2 0 1) of the wurtzite-type ZnO (JCPDS no.

36-1451).

It

indicates

that

the

core-shell

hierarchical

structure

of

CC/ZnO@C@NiO contains the crystalline wurtzite-type ZnO and the cubic NiO 14


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obtained by post thermal treatments, which are well agreement with the analysis result of TEM.

In order to investigate the crystalline state of the precursor of NiO, the XRD pattern of the powder of NiO precursor was collected. The result in Figure S6 demonstrates the pattern is corresponding to 4Ni(OH)2•NiOOH (JCPDS no. 06-0044), indicating that the precursor deposited on CC/ZnO@C in the chemical bath process should be 4Ni(OH)2•NiOOH. Considering the reacting agents used in the current work, the reactions may occur as followed25-27 :

NiH O NH + 2OH → NiOH + 6 − xH O + xNH 1 10NiOH + S O → 24NiOH ∙ NiOOH + 2SO + 2H 2 Ammonia may easily combine with Ni2+ and solvent molecules to form a complex ion of [Ni(H2O)6-x(NH3)x]2+, then reacts with hydroxyl ions to produce Ni(OH)2. In presence of the oxidant potassium peroxydisulfate, Ni(OH)2 may be partially oxidized to NiOOH, which exists as 4Ni(OH)2•NiOOH, and further converts into cubic type NiO. In addition, we found this special reaction system, containing Ni2+, ammonia, and potassium peroxydisulfate, was prone to the easy deposition of NiO film on the substrates, compared with that of Ni2+ and ammonia. It is probably due to the formation of the composite of Ni(OH)2 with NiOOH.

15



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Figure 5. XRD patterns of the carbon cloth; CC/NiO; CC/ZnO; CC/ZnO@C@NiO. In terms of specific elemental composition and the chemical bonding state of the CC/Zn@C@NiO CSNAs, the XPS measurement was recorded, and the corresponding results are demonstrated in Figure 6. The XPS survey spectrum of CC/ZnO@C@NiO contains O, C, Ni, a small amount of Zn, without any other impurities. As one knows the effective range of XPS detection is only a few nanometers, the element of C seen in Figure 6a mainly comes from the carbon layer coated on ZnO NAs since the carbon cloth substrate was completely covered by ZnO@C@NiO CSNAs. The appearance of Zn elements from ZnO nanoneedles means that the NiO nanosheets are very thin and their interconnected nature generates the porous structure of NiO nanosheets, which the X-ray may reach the surface of ZnO NAs in the inner of the core-shell structure. As illustrated in Figure 6b, the binding energies at 1021.1 and 1044.4 eV are ascribed to Zn 2p3/2 and Zn 2p1/2, respectively.

16


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Figure 6. XPS survey spectra of ZnO@C@NiO NAs. (a) XPS wide-scan spectra; (b−d) high-resolution XPS spectra for Zn, C and Ni. In the C 1s spectrum (Figure 6c), the peak at 284.7 eV is assigned to the delocalized sp2-hybridized carbon or graphite-like C-C bonding of the carbon layer coating on the ZnO NAs, and the other two peaks at binding energies of 285.8 and 288.2 eV are corresponding to species of C-O and C=O produced in the process of chemical bath and annealing.27-29 For the Ni 2p spectrum in Figure 6d, one strong peak is located at 854 eV due to Ni2+ in Ni-O bonds and a weak peak at 855.8 eV is corresponding to nonlocal screening and affection by surface states of NiO, such as hydroxylated NiO. And two distinct shakeup satellite peaks (indicated as “Sat”) are found at the binding energies of 861.1 and 879.6 eV.30-34 These are the typical characteristic peaks of Ni2+ species, confirming the formation of the NiO shell.28, 34 The investigated XPS results indicate that the sample has a mixed composition, containing ZnO and NiO, among them, the NiO provides active sites for the reversible Faradaic redox reactions. 17



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3.2 Electrochemical Properties To demonstrate the electrochemical performance of the prepared binder-free electrodes, the free-standing electrodes of CC/ZnO@C@NiO and CC/ZnO@NiO were investigated in a three-electrode system with 3 M KOH as electrolyte.

Figure 7a shows the typical cyclic voltammograms (CVs) of the CC/ZnO@C@NiO electrode with different scan rates ranging from 2 to 40 mV s−1. The well-defined redox peaks in all the CV curves indicate that the electrochemical capacitance of the CC/ZnO@C@NiO electrode is mainly governed by Faradic redox reactions. Since NiO is the main active material for Faradic redox reactions, as illustrated in Figure 7a , the anodic peak is mainly attributed to the conversion from NiO to NiOOH, while the cathodic peak is the reverse conversion from NiOOH to NiO. The complete real reaction is demonstrated as follows35.

NiO + OH − e ↔ NiOOH 3 When the scan rates increase, the peaks of anode move to the positive potential and the peaks of cathode shift to negative potential, due to the redox reaction controlled by the charge transfer dynamics.36 With the scan rate increasing from 2 to 40 mV/s, the position of the anodic peak shifts from 0.485 to 0.610 V, and the CV curves can keep a similar redox shape compared to original shape. It implies that the CC/ZnO@C@NiO electrode exhibits excellent rate performance. However, compared with CC/ZnO@C@NiO, the anodic peak of CC/NiO starts to destroy at 5 mV/s and disappears when the scan rate increases to 10 mV/s (Figure S7a), which indicates the

18


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CC/NiO electrode has the poor rate performance due to the slow electron/charge diffusion rate without the inner core of ZnO@C nanorods arrays. It further reflects that the ZnO@C nanorods arrays in the CC/ZnO@C@NiO electrode play an important role in improving the electron/charge transport rate. The high conductivity of the CC/ZnO@C can be verified in the following section.

Figure 7. CV curves (a) at different scan rates and CP curves (b) at different discharge current densities of the CC/ZnO@C@NiO CSNAs. Comparison in electrochemical properties of CC/ZnO@NiO CSNAs and CC/ZnO@C@NiO CSNAs: (c) CV curves at 5 mV/s, (d) Specific capacity at different current densities from 1 to 16 mA/cm2, Error bars: Standard deviation (CC/ZnO@C@NiO: 17.5 C/g; CC/ZnO@NiO: 19



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14.6 C/g). (e) Impedance Nyquist plots at open circuit potential. (f) The cycling performance at the current density of 4 mA/cm2. To better study the electrochemical performance of the as-prepared hierarchy structure electrode, the measurement of galvanostatic charge-discharge was implemented under 0-0.5 V at different current density ranging from 1 mA/cm2 to 16 mA/cm2. The result is exhibited in Figure 7b. From the curves of charge-discharge, it’s apparent that there exists a voltage platform from 0.41 V to 0.5 V, indicating that the redox reaction plays a key role in the whole charge-discharge process. The specific capacity (C/g) was calculated from the charging/discharging profiles based on Equation 4:37 C=

& ∙ ∆( 4 )

Where I, ∆( and m are the discharging current (A), the discharging time (s) and mass of NiO, respectively. Notably, the capacity of the NiO in CC/ZnO@C@NiO was as high as 677 C/g at current density of 1.43 A/g.

The CV curves of the CC/ZnO@C@NiO and the CC/ZnO@NiO electrodes at the scan rate of 5 mV/s were shown for comparison in Figure 7c. Evidently, the CC/ZnO@C@NiO has a larger integrated area of the CV curve than the CC/ZnO@NiO does, suggesting that the former owns the higher electrochemical energy storage capacity. This may be attributed to the good electrical conductivity of carbon layer around ZnO NAs, which provides well connection between NiO and ZnO during the process of chemical bath reaction and calcination. In Figure S7b, compared to integrated area of CV curves of CC/ZnO@C@NiO, CC/ZnO@NiO and CC/NiO, the CV curves of CC and CC/ZnO are almost a straight line with negligible 20


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area, which indicate the contribution of ZnO to total capacity is very low.

The relationships between the calculated specific capacity and current density of both electrodes are presented in Figure 7d. With the increase of current density, the specific capacity for both CC/ZnO@C@NiO and the CC/ZnO@NiO CSNAs electrodes gradually decreases, which is due to the incremental voltage drop and inadequate active material involved in redox reaction at a higher current density. Even so, the NiO in CC/ZnO@C@NiO CSNAs electrode delivers the high specific capacity of 677.0, 572.0, 479.3, 384.3 and 285.3 C/g (0.48, 0.40, 0.385, 0.265 and 0.20 C/cm2) at current density of 1.43, 2.86, 5.71, 11.43 and 22.9 A/g (1, 2, 4, 8, 16 mA/cm2), respectively. On account of the core-shell structure, the high specific capacity of CC/ZnO@C@NiO at large current density is superior to many reported NiO related materials.38-41 The three-dimensional ZnO@C nanorods arrays regarded as a scaffold provide dominantly a high-speed channel for the transport of electrons and ions due to the conducting cover of carbon layer introduced.

EIS measurement was implemented to evaluate the electrochemical behavior of the CC/ZnO@C@NiO and CC/ZnO@NiO CSNAs electrodes. As shown in Figure 7e, the homologous Nyquist plots were obtained in the frequency range from 0.01 to 100 kHz, which shows the evident semicircles at high-frequency for three electrodes. In high-frequency region, the semicircle of the plots represents the charge-transfer process between working electrode and electrolyte, which can indirectly reflect the conductivity of the electrode.42 It can be seen that the diameter of the semicircle

21



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containing the presented arc for CC/ZnO@C@NiO is smaller than that for CC/ZnO@NiO, indicating the higher electron conductivity of CC/ZnO@C@NiO due to the introduction of carbon layer between ZnO and NiO layers. In the low frequency area, the slope of the plot indicates the Warburg impedance. Both two electrodes show evident straight lines, suggesting valid proton and electrolyte diffusion. While, the charge-discharge resistance (Rct) of CC/ZnO@C@NiO is 19.9 Ω/cm2 (obtained from the fitting circuit model with ZSimpWin software)43 which is smaller than CC/ZnO@NiO (27.5Ω/cm2). The result further confirms the importance of carbon layer introduced into the integrated electrode of CC/ZnO@NiO. These EIS curves suggest that the direct growth of the ZnO@C@NiO CSNAs possesses good electrical conductivity due to the presence of highly conductive carbon layer.

In order to better understand the role of the ZnO NAs and carbon layer in multilayer electrode, CV, CP and EIS measurements of the CC, CC/ZnO, CC/NiO and the CC/ZnO@NiO were carried out in 3 M KOH electrolyte. As shown in Figure S7b, the integrated CV areas of the CC and CC/ZnO are nearly negligible compared with other hybrid electrodes based on NiO, indicating the capacity of the CC/ZnO@C@NiO CSNAs electrode mainly derives from the NiO nanosheets. Meanwhile, as denoted in the Figure S7c and d, the discharge time approaching zero and the little resistance of the CC and CC/ZnO indicate that the CC, CC/ZnO almost have no contribution to the total capacity. Therefore, ZnO and ZnO@C dominantly play an important role in supporting substrate for NiO loading, which is also helpful for the fast charge / ion transfer due to the good conductivity. The CC/NiO nanosheets have the lowest 22


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specific capacity and largest the charge-transfer resistance (52.8Ω/cm2), suggesting the outstanding electrochemical properties of the CC/ZnO@C@NiO CSNAs are owning to the good conductivity of ZnO skeleton and carbon layer, and high surface area of the well-designed hierarchical core-shell structure.

The cycling capability of the CC/ZnO@C@NiO was conducted by CP for 5000 cycles at 8 mA/cm2 within a potential window of 0-0.5 V. As is shown in Figure 7f, the capacity of the NiO in CC/ZnO@C@NiO electrode can retain 71% of the initial capacity, whereas the capacity of the NiO in CC/ZnO@NiO only keeps 21% after 5000 cycles at the same current density. Moreover, as demonstrated in Figure 8a, the complete skeleton of CC/ZnO@C@NiO CSNAs still remained after 5000 cycles, just with slightly shrinked nanorods, compared with that in Figure 2f. It is due to the partial corrosion of the inner core of ZnO under alkane solution for a long time. But for CC/ZnO@NiO CSNAs, the core-shell skeleton is almost completely destroyed after 5000 cycles under the same test condition because of the collapse of inner core of ZnO NAs without protection of carbon layer (shown in Figure 8b). The fast corrosion of ZnO results from the fast solvation of ZnO in alkaline, which is because of the poor three dimensional architecture of NiO shell deposited directly on ZnO NAs. To address the importance of carbon coating on ZnO NAs, the SEM images of CC/ZnO NAs and CC/ZnO@C NAs after 5000 cycles are also given as Figure S8a and b. As observed clearly, the ZnO NAs on CC (Figure S8a) are completely damaged and disappear, due to the corrosion in KOH electrolyte. However, with the protection of carbon layer on the surface, the nanorod ZnO@C can remain well after 5000 cycles 23



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(Figure S8b). Meanwhile, the elemental mapping of ZnO@C nanorod inserted into Figure S8b shows the existence of C, O and Zn. Therefore, the strategy to protect ZnO NAs with conducting carbon layer is successfully proved. Therefore, it is evident that the cycle performance of the CC/ZnO@C@NiO is improved significantly due to the existence of the carbon layer.

Figure 8. SEM images of the CC/ZnO@C@NiO CSNAs (a) and CC/ZnO@NiO CSNAs (b) after 5000 cycles. There are many electrode materials (such as: Co3O4@Pt@MnO2,44 Co3O4 nanofilm45 and so on) which has been studied recently. Table 1 summarizes the electrochemical performance of the related ZnO or nickel-based materials for supercapacitors reported in literature. With comparison, the CC/ZnO@C@NiO electrode shows the higher specific capacity than most of the reported electrodes and the better cycling stability than those of the ZnO@MoO3 nanocables,46 ZnO@Mn3O4 arrays,47 ZnO@Ni(OH)2 arrays,23 ZnO@Ni3S2 arrays.6 The superior performance of CC/ZnO@C@NiO in our work results from the unique core-shell structure and the carbon layer between ZnO and NiO layers

Table 1. Comparison in electrochemical performance of CC/ZnO@C@NiO with

24


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ZnO- or Nickel-based materials for SCs in literature. Material

Specific capacity (C/g)

Stability

Ref.

Nanoflower NiO Porous NiO nanosphere Nanosheets@miscrosphrical NiO NiO hollow nanospheres Porous NiO nanotube arrays NiO/Ultrathin graphene

152.4 (381 F/g) at 1 A/g 426 (710 F/g) at 1 A/g 323.1 (718 F/g) at 2 A/g 388.5 (555 F/g) at 1 A/g 270 (675 F/g) at 2 A/g 170 (425 F/g) at 2 A/g 287.4 (643.2 F/g) at 5.8 A/g 244.4 (611 F/g) at 2 A/g 165 (275 F/g) at 3 A/g 1014 (2028 F/g) at 10 A/g 764.5 (1529 F/g) at 2 A/g 120.5 (241 F/g) at 5 mV/s 777.6 (1296 F/g) at 1 A/g 224.3 (448.5 F/g) at 0.3 A/g 823.5 (1830 F/g) at 2 A/g 448.0 (896 F/g) at 5 A/g 1033.2 (1722 F/g) at 8.6 A/g 806.8 (2017 F/g) at 2.5 A/g 677 at 1.43 A/g

90% (400 cycles ) 98% (1700 cycles ) 95% (1000 cycles ) 90% (1000 cycles) 93% (10000 cycles) 79% (2000 cycles)

38 48 49 50 51 52

99% (400 cycles)

41

89% (5000 cycles) 93% (1000 cycles)

35 45

68% (500 cycles)

23

42% (2000 cycles)

6

65% (1000 cycles)

41

86.9% (10000 cycles)

53

74% (500 cycles)

54

70% (1000 cycles)

55

98% (1000 cycles)

56

94.8% (1200)

57

94.5% (3000)

58

71% (5000 cycles)

This work

ZnO-NiO composite particles TiO2/NiO arrays ZnO@Mn3O4 arrays ZnO@Ni(OH)2 arrays ZnO@Ni3S2 arrays ZnO@MoO3 nanocables NiO nanosheets on nickel foam NiO particles ZnO@Ni(OH)2 nanoflakes ZnCo2O4/H: ZnO NRs H-Ni/NiO Co-Ni/Co3O4-NiO CC/ZnO@C@NiO NAs

To assess the future prospect of the all-solid-state asymmetric device in aspect of the CC/ZnO@C@NiO CSNAs for energy storage, an all-solid-state asymmetric device was assembled with the CC/ZnO@C@NiO as the cathode and commercial graphene as the anode separated by PVA-KOH as solid electrolyte and separator, signed as CC/ZnO@C@NiO CSNAs//graphene. The Figure S9 shows that the commercial 25



Page 26 of 36

graphene delivers 201.6, 191.8, 176.5 and 165 C/g at 1, 2, 5, 10 A/g, respectively. According to the charge balance principle, the optimum mass ratio of the positive and negative electrode can be calculated by the Equation 5 as below: ) +_∆-_ 1 = = 5 ) + ∆- 3.36 Figure 9a presents the typical CP curves of the asymmetric supercapacitor at various current densities ranging from 0.5 to 4 A/g. No obvious IR drop is observed for any of these curves, indicating a low internal resistance of the asymmetric device which is benefit to superior cycling performance. The CV curves at different scan rates are shown in Figure 9b, in which the area surrounded by CV is enlarged with the increase of the scan rates. This positive relation indicates the rapid diffusion of the ions in polymer electrolyte.59 Figure 9c shows the cycling performance carried out by CP at 4 A/g for 10000 cycles. The capacitance of the supercapacitor remains about 87.5% after cycling 10000 times, indicating the outstanding long-term cycling stability. The cycling performance is superior to most reported asymmetric devices including NiO//Activated carbon (93% after 800 cycles),59 Graphene/nanowire//Graphene (79% retention after 2000 cycles),60 Ni(OH)2//Activated carbon (82% retention after 1000 cycles),61 Ni-Co oxide//Activated carbon (85% retention after 2000 cycles).62 Also, from the Figure S10, we can know that the asymmetric device during cycling test can keep good stability at room temperature, but get worse at a higher temperature, due to the ageing and solidification of PVA gel.

26


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Figure 9. (a) CP curves, (b) CV curves and (c) cycling performance of the device at the different current density; (d) Comparison of energy and power densities of the CC/ZnO@C@NiO CSNAs//Commercial graphene solid-state device with Ni-based material; (e) The brightness of LED bulb at different times. The energy densities and power densities of our all-solid-state supercapacitor compared with other Ni-based asymmetric supercapacitors were shown in the Ragone plot (Figure 9d). The energy density (E) and power density (P) in a constant current charging/discharging process are calculated by Equation 6:37 E=/

1

2

&0 3(, )

1 1 &0 5= / 3( 6 ( 2 ) 27



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where I, V, m and t are the current, the discharging voltage range (V), total mass of the active materials in both positive and negative electrodes, and the discharging time, respectively. The energy density of the this all-solid-state device decreases from 35.7 to 16.0 Wh/Kg as the power density increase from 380.9 to 2704.2 W/Kg, which is higher than many reported asymmetric devices, such as NiO//Carbon (11.6 Wh/Kg at 28 W/Kg),63 CNT@NiO//PCPs (25.4 Wh/Kg at 400 W/Kg),64 NiO//rGO (23.3 Wh/Kg at

151

W/Kg),65

Ni-Co

oxide//AC

(9.5

Wh/Kg

at

900

W/Kg),66

Ni(OH)2/Graphene//Graphene (11.2 Wh/Kg at 1900 W/Kg),67 3DCS/P3//3DCS (4.37 Wh/Kg at 50.38 W/Kg),68 3D graphene/CoMnO4//AC (26.8 Wh/Kg at 532 W/Kg),69 except for α-Fe2O3//Co3O4@Au@CuO ( 20.6 Wh/Kg at 40400W/Kg).70 Besides, the Figure 9e shows a circuit design composed of a red LED and two-tandem asymmetric devices. With the subsequent decrease in voltage, the LED can light for 5 min and then become weaker in brightness. This phenomena indicate the potential application of our all-solid-state device. 4. Conclusion In summary, a novel hierarchical structure of CC/ZnO@C@NiO was obtained via a simple hydrothermal, chemical bath and annealing procedure. A carbon layer can efficiently protect inner ZnO cores from chemical corrosion in alkane solution, which also contributes to the enhancement of conductivity and electrochemical performance of the as-prepared hierarchical configuration nanoarrays of CC/ZnO@C@NiO electrode. The NiO in CC/ZnO@C@NiO CSNAs electrode exhibits high specific capacity (677 C/g at 1.43 A/g) and high cycling stability (capacity remain 71% after 28


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5000 cycles). The high energy density and power density of the all-solid-state asymmetric supercapacitor can reach 35.7 Wh/Kg (at 380.9 W/Kg) and 2704.2 W/Kg (at 16.0 Wh/Kg) with excellent cycling stability (capacitance remain 87.5% after 10000 charge-discharge cycles). This work may provides an open idea to fabricate the hierarchical nanostructure of free-standing electrodes for small, light-weight and flexible electronic devices.

ASSOCIATED CONTENT Supporting information BET of CC/ZnO@C and CC/ZnO@C@NiO, SEM of CC/ZnO@NiO, SAED of NiO nanosheets coating on ZnO@C, EDS linear sweep and mapping of ZnO@C@NiO, XRD of NiO precursor, CV curves of CC/NiO and comparison of electrochemical

tests

of

CC,

CC/ZnO,

CC/NiO,

CC/ZnO@NiO

and

CC/ZnO@C@NiO, SEM images of CC/ZnO and CC/ZnO@C after 5000 cycles, CP curves of commercial graphene, cycling performance of supercapacitor device at different temperatures. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected] (W. Lei); [email protected] (Q. Hao). *Fax: +86-25-84315190; Tel: +86-25-84315190 Notes The authors declare no competing financial interest. 29



Page 30 of 36

ACKNOWLEDGMENTS The work was supported by the National Natural Science Foundation of China (Nos. 21576138, 51572127), China-Israel Cooperative Program (2016YFE0129900), Program for NCET-12-0629, Natural Science Foundation of Jiangsu Province (BK20160828), Post-Doctoral Foundation (1501016B), Six Major Talent Summit (XNY-011), and PAPD of Jiangsu Province, and the program for Science and Technology Innovative Research Team in Universities of Jiangsu Province, China. We also thank Dr. Huaping Bai and Dr. Wanying Tang at Analysis and Test Center Nanjing University of Science and Technology.

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Table of Contents Graphic

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Three-Dimensional hierarchical structure ZnO@C@NiO on carbon

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