Template-assisted synthesis of NiP@CoAl-LDH nanotube arrays with superior electrochemical performance for supercapacitors

Accepted Manuscript Title: Template-assisted synthesis of NiP@CoAl-LDH nanotube arrays with superior electrochemical performance for supercapacitors Author: Senlin Wang Zongchuan Huang Rui Li Xuan Zheng Fengxia Lu Taobin He PII: DOI: Reference:

S0013-4686(16)30850-7 http://dx.doi.org/doi:10.1016/j.electacta.2016.04.051 EA 27086

To appear in:

Electrochimica Acta

Received date: Revised date: Accepted date:

12-1-2016 27-3-2016 10-4-2016

Please cite this article as: Senlin Wang, Zongchuan Huang, Rui Li, Xuan Zheng, Fengxia Lu, Taobin He, Template-assisted synthesis of NiP@CoAl-LDH nanotube arrays with superior electrochemical performance for supercapacitors, Electrochimica Acta http://dx.doi.org/10.1016/j.electacta.2016.04.051 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.

Template-assisted synthesis of NiP@CoAl-LDH nanotube arrays with superior electrochemical performance for supercapacitors

Senlin Wang*, Zongchuan Huang, Rui Li, Xuan Zheng, Fengxia Lu, Taobin He

College of Materials Science & Engineering, Huaqiao University, Xiamen, Fujian, 361021, People’s Republic of China

*Corresponding author. Fax: +86-595-22693746. E-mail address: [email protected]

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Abstract A novel NiP@CoAl-LDH nanotube arrays (NiP@CoAl-LDH NTAs) are successfully designed on Ni foam via template-assisted electrodeposition process. The unique nanotube arrays can provide a short diffusion path for electrolyte ions and a high utilization rate for electrode materials. As a consequence, the obtained NiP@CoAl-LDH NTAs electrode presented the superior electrochemical performances, which demonstrated an areal specific capacity of 0.67 C cm-2 (556 C g-1) at 1 mA cm-2 and a high capacity retention of 74.1% at 20 mA cm-2, much higher than that of NiP NTAs and CoAl-LDH electrodes. The asymmetric supercapacitor device (ACS device) also displayed predominant electrochemical properties, such as high specific capacity of 156.1 C g-1 at 2 mA cm-2 and energy density of 37.18 Wh kg-1 at a power density of 0.45 kW kg-1. Besides, the specific capacity of ACS device maintained 95.50 % of the initial specific capacity at 6 mA cm-2 after 4000 cycles, indicating its potential applications in long-term cycle life supercapacitor. These results illustrate the promise of NiP@CoAl-LDH NTAs as electrode for supercapacitor.

Keywords: Template-assisted electrodeposition; CoAl layered double hydroxide; NiP nanotube arrays; supercapacitor

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1.

Introduction

Supercapacitors, as a novel energy storage device, have attracted a wide interest due to their fast charging property, high power density and long cycle life [1-3]. Early research mainly focused on the electrical double-layer capacitors (EDLCs) such as carbon species, whose capacitance stems from the interface between electrolyte and electrode [4, 5]. However, the low energy density and poor rate capability greatly limit their application [6, 7]. Recently, redox-based supercapacitors have shown the potential application for energy storage device due to their high specific capacity and reversible redox reaction [8, 9]. In this regard, transition metal oxides/sulfides/hydroxides (RuO2 [10], Ni3S2 [11], and Co(OH)2 [12]) and conducting polymers (PANI [13] and PPy [14]) have been reported for supercapacitve materials. Recently, layered double hydroxides (LDHs) have also drawn great interest for the application of supercapacitors because of their layered structure with abundant channels between layers and the existence of transition metals, which can simultaneously take advantage of electrical double layered capacitance and Faradaic capacity [15]. However, the practical use of LDHs was greatly limited by the low specific capacity as a single electrode material, which could not meet the requirements of supercapacitors with high power density. The major strategy to overcome this problem is constructing a core/shell structure with different Faradic electrode material. In the past few decades, many groups were committed to metal-based oxides/hydroxides core/shell structure because of their high specific capacity [16-19]. However, most of these reported materials are limited by its low capacity retention. Noteworthy, metal-based phosphides are n-type semiconductors, which was kinetically favorable to provide a fast electron transport at high power density and maintain a high capacity retention [20]. Besides, metal-based phosphides present the metalloid characteristics and high electrical conductivity, indicating its potential application in supercapacitors [21, 22]. Recent researches have also shown that transition metal phosphides can be used as Faradic electrode materials [23-25]. Thus, it is urgent to develop the metal-based phosphides as core-shell structure for 3

supercapacitors. Among all the phosphides, nickel phosphides are an interesting kind of electrode material due to their potential applications and remarkable properties [26]. Herein, we synthesized NiP@CoAl-LDH NTAs electrode via template-assisted electrodeposition process, in which NiP nanotube and CoAl-LDH nanosheets were considered as core and shell, respectively. The unique nanotube arrays can provide a short diffusion path for electrolyte ions and abundant active sites for Faradaic process, resulting in a high utilization rate for electrode materials. Besides, each nanotube contacted with the other nanotubes, making the strong connection between nanotubes, and will eventually lead to the long-term cycle life. As a consequence, the as-prepared NiP@CoAl-LDH NTAs electrode exhibited several desirable electrochemical performances for supercapacitor: a high specific capacity (areal specific capacity of 0.67 C cm-2 and mass specific capacity of 556 C g-1 for three-electrode cell at 1 mA cm-2 with a high retention of 74.1 %, mass specific capacity of 156.1 C g-1 for asymmetric supercapacitor device at 2 mA cm-2) and energy density (37.18 Wh kg-1 at a power density of 0.45 kW kg-1), and an excellent cycle stability (95.50 % retention at 6 mA cm-2 after 4000 cycles). 2.

Experimental sections

All chemicals used in this study were of analytical grade without further purification. Ni foam (1 cm×1 cm) was pretreated with 5% HCl solution, ethanol and deionized water for 15 min each other before experiment. Electrodeposited process was performed on a CHI 660E electrochemical workstation in a simple three-electrode configuration with a Pt plate (1 cm×1 cm) as the counter electrode and Ag/AgCl as the reference electrode. The details of the preparation of NiP@CoAl-LDH NTAs electrode are described as follows:

1) ZnO nanorod arrays (ZnO NRAs) template was coated on the pretreated Ni foam by a simple wet-chemical process [27]. Briefly, a seed layer was first formed by immersing Ni foam into 0.5 M KMnO4 for 30 min. A volume of 100 mL solution mixed with 0.01 mol zinc nitrate hexahydrate and 0.01 mol 4

hexamethylenetetramine (HMT) was then prepared and stirred at room temperature for 30 min. Subsequently, 80 mL of the above solution was transferred into a 100 mL Teflon-lined stainless steel autoclave with 5 pieces of seeded Ni foam and hydrothermally reacted at 95 °C for 12 h. The obtained substrate was then washed with deionized water and dried in air at 60 °C.

2) ZnO@NiP nanorod arrays (ZnO@NiP NRAs) was synthesized by electrodepositing of NiP layer on the surface of ZnO NRAs at a current density of 0.5 mA cm-2 in a solution containing 0.007 M NaH2PO2·H2O+0.002 M C6H5Na3O7·2H2O + 0.0175 M NiSO4·6H2O (pH was adjusted to 6.0 by using Na2CO3) at 50 °C for 2 h [28]. To remove ZnO template, the obtained ZnO@NiP NRAs electrode was immersed into 3 M NaOH solution for 3 h and followed by washing with deionized water for three times. Finally, the fabricated NiP nanotube arrays (NiP NTAs) was dried at vacuum oven under the protection of N2. 3) NiP@CoAl-LDH NTAs were finally fabricated by the electrodeposition of CoAl-LDH on the surface of NiP NTAs using cyclic voltammetry (CV) method. The electrolyte for electrodeposition was composed of 0.0075 M Co(NO3)2, 0.0025 M Al(NO3)3 and 0.1 M KNO3, and the process was performed for 20 cycles at 50 mV s-1 in the voltage range from -1.1 to 0 V. Then, the obtained NiP@CoAl-LDH NTAs electrode was washed with deionized water and dried at vacuum oven under the protection of N2. The loading of NiP and NiP@CoAl-LDH were approximately 0.9 and 1.2 mg cm-2, respectively. For comparison, CoAl-LDH electrode was electrodeposited on Ni foam directly by applying the same experimental conditions with the synthesis of NiP@CoAl-LDH NTAs, and the mass loading of CoAl-LDH on Ni foam was calculated to be 0.9 mg cm-2. 2.1. Structure characterization Crystallite structures of the as-prepared products were characterized by X-ray diffraction (XRD, Rigaku SmartLab) using a Cu Kα (λ=1.5418 Å) radiation. The morphology and microstructure of the synthesized 5

materials were determined by field emission scanning electron microscopy (FESEM, SU-8000), transmission electron microscopy (TEM, JEM-2100) and X-ray photoelectron spectroscopy (XPS, ESCALAB-250). 2.2. Electrochemical Measurements The electrochemical measurements of the obtained single electrode were carried out in 2 M KOH aqueous solution using three-electrode cell. A platinum plate (3 cm×3 cm) was used as the counter electrode and Hg/HgO electrode was used as the reference electrode. Cyclic voltammograms (CV) and galvanostatic charge-discharge (GCD) measurements were recorded on an electrochemical workstation (CHI 660E) to evaluate the electrochemical behaviors. The average specific capacity determined from galvanostatic charge-discharge (GCD) measurements can be calculated by eqn (1) as follow: 𝐶𝑎 =

𝑖×𝑡 𝑖×𝑡 or 𝐶𝑚 = 𝑆 𝑚

(1)

Here Ca is the areal specific capacity (C cm-2), Cm is the mass specific capacity (C g-1), i is the discharge current (A), t is the discharge time (s), S is the surface area (cm2) and m is the mass (g) of single electrode. 2.3. Fabrication and characterization of ACS devices ASC device was fabricated by taking NiP@CoAl-LDH NTAs and activated carbon (total mass: 3.84 mg) as positive and negative electrodes, respectively. Prior to the assemble of the ASC devices, activated-carbon (AC) electrode was prepared by mixing activated carbon, polytetrafluorene-ethylene (PTFE) and acetylene black in ethanol with mass ratio (%) of 80:10:10. The obtained mixture was ultrasonicated for 1 h and coated on Ni foam. After that, the Ni foam with AC coating was pressed and dried at 60°C for at least 12 h. According to the charge balance (eqn (3): q+=q-) as follow, the mass ratio of positive electrode to negative electrode was calculated to be 0.45. + 𝑞 + = 𝑚+ × 𝐶𝑚 − 𝑞 − = 𝑚− × 𝛥𝑉 − × 𝐶𝑠𝑝

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(2)

+ 𝑚− 𝐶𝑚 = − 𝑚+ 𝛥𝑉 − × 𝐶𝑠𝑝

(3)

+ Here, 𝑞 + , 𝑚+ and 𝐶𝑚 represent the charge (C), mass (g) and specific capacity (C g-1) of positive electrode; − 𝑞 − , 𝑚− , 𝛥𝑉 − and 𝐶𝑠𝑝 represent the charge (C), mass (g), voltage range (V) and specific capacitance (F g-1)

of AC electrode in three-electrode cell. For ASC devices, the electrochemical properties were also investigated on CHI 660E electrochemical workstation by using cyclic voltammograms (CV), galvanostatic charge-discharge (GCD) and electrochemical impedance spectroscopy (EIS) measurements. The cycling performance was inspected through a LAND-CT2001A system. The average specific capacity can be calculated from eqn (4) as follow: 𝐶𝐴𝑆𝐶 =

𝐼×𝑡 𝑀

(4)

Here CASC is the mass specific capacity (C g-1) of ASC devices, I is the discharge current (A), t is the discharge time (s) and M is the total mass (g) of positive and negative electrode. The energy and power density of ASC devices were estimated from the following equations (eqn (5) & (6)): 𝐸 = ∫ 𝐼 × 𝑉(𝑡)𝑑𝑡

(5)

𝐸 𝛥𝑡

(6)

𝑃=

Here E is energy density (Wh kg-1), P is the power density (W kg-1), ΔV is the cell voltage (V) and t is the discharge time (s) of ASC devices. 3.

Results and discussion

3.1. Material characterization A procedure of the fabrication is shown in Fig. 1, and the details are illustrated in the Experimental Section. Typically, ZnO NRAs and ZnO@NiP NRAs are sequentially fabricated and follow by the dissolution of ZnO NRAs template to obtain NiP NTAs. Then, the NiP@CoAl-LDH NTAs were synthesized

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by electrodeposition of CoAl-LDH nanosheets on NiP NTAs. Fig. 2a shows the XRD pattern of ZnO NRAs and NiP@CoAl-LDH NTAs. The obtained ZnO NRAs present a typical wurtzite-type structure (JCPDS: 36-1415). For NiP@CoAl-LDH NTAs, no peaks of CoAl-LDH and NiP are observed except for Ni foam, which may be due to the low mass loading of NiP@CoAl-LDH NTAs and the poor crystallize of CoAl-LDH and NiP. To prove the structure of the as-prepared products, the powder of the samples were scraped from the Ni foam for XRD tests and the result was shown in Fig. S1 (Supporting Information). Compared with ZnO NRAs, ZnO@NiP NRAs shows an additional peak at 45°, which can be attributed to (111) plane of NiP phase [29, 30]. For NiP@CoAl-LDH NTAs, the peaks at 11.3°, 22.6° and 61.7° can be indexed to the hydrotalcite-like structure (JCPDS: 51-0045) of CoAl-LDH phase. The weak and broad diffraction peaks indicate the poor crystallinity of CoAl-LDH and NiP. Apart from the peaks of NiP and CoAl-LDH phases, the other peak of NiP@CoAl-LDH NTAs can be assigned to the remaining ZnO phase. To further prove the existence of NiP, XPS spectra of NiP NTAs is shown in Fig. 2b and c. The peaks center at 858.1 and 875.6 eV for Ni 2p (Fig. 2b) can be assigned to Ni 2p3/2 and Ni 2p1/2 for metallic Ni, respectively. For the Fig. 2c, the high resolution spectrum of P 2p shows a peak located at132.7 eV and the peak at 139.2 eV can be attributed to the Zn 3s region from the remaining ZnO. These results are consistent with the binding energies for metal-based phosphides reported before [28, 31]. Fig. 2d shows TEM images of NiP NTAs with a wall thickness of about 40 nm. It has been reported that the existence of P can drastically reduce the size of metal-based nanoparticles, and eventually resulted in the poor crystallinity [28]. Thus, the as-prepared NiP presents a weak crystallinity. This can be confirmed by HRTEM (inset in Fig. 2d) images of NiP NTAs that no clear lattice fringes were observed. To further prove the existence of CoAl-LDH in NiP@CoAl-LDH NTAs and investigate the structure of NiP@CoAl-LDH NTAs, EDS analysis and TEM images of NiP@CoAl-LDH NTAs are shown in Fig. 3. In the EDS profile of NiP@CoAl-LDH NTAs (Fig. 3a), the clear peaks of Ni, P, Co, Al and O confirm the 8

successful co-deposition of Ni, P on ZnO NRAs and Co, Al, O on NiP NTAs. Quantitative analysis shows 48.96 wt% Ni, 1.78 wt% P, 7.48 wt% Co, 2.44 wt% Al and 39.35 wt% O in the NiP@CoAl-LDH NTAs. Fig. 3b-e show the TEM images and selected area electron diffraction (SAED) pattern of NiP@CoAl-LDH. TEM observation of NiP@CoAl-LDH under various magnifications (Fig. 3b, c and e) indicate that CoAl-LDH nanosheets deposited on NiP NTAs surface are highly uniform and interconnected with each other. In addition, TEM images (Fig. 3b and c) at the lower magnification show a typical core-shell nanotube structure of NiP@CoAl-LDH. Such unique nanostructures can provide a short diffusion path for electrolyte ions and abundant active sites for Faradaic process. Fig. 3e presents the HRTEM image of NiP@CoAl-LDH nanotube with a typical lattice fringe of CoAl-LDH. The fringe spacing between lattice fringes was estimated to be 0.78 nm, matching well with the lattice spacing of (003) plane of CoAl-LDH. SAED pattern of external shell in Fig. 3d shows a set of diffraction ring, suggesting the polycrystalline nature of CoAl-LDH nanosheets. Furthermore, the diffraction rings from inside to outside can be assigned to the (015) and (110) planes of CoAl-LDH, which further prove the existence of CoAl-LDH in NiP@CoAl-LDH NTAs. Based on the above discussions, we can come to the conclusion that a core-shell nantube arrays structure has been successfully fabricated with NiP nanotube as the core and interconnected CoAl-LDH nanosheets as the shell. The surface morphology of the products was further characterized by FESEM. Fig. 4A and D show the FESEM images of ZnO NRAs with different magnifications. Apparently, the ZnO nanorod was homogeneously coated on Ni foam surface and separate from each other. A typical FESEM image of hexagonal ZnO nanorods is shown in the inset in Fig. 4D, which presents a diameter of 200~500 nm and an average height of 2~3 um. Fig. 4B and E show the FESEM image of NiP NTAs. It is clear that NiP NTAs possess a nanotube structure. These nanotubes are almost vertical to Ni foam and strongly connected with each other, which was considered to be responsible for the long-term cycling stability. To further identify the 9

nanotube structure, a magnified FESEM of NiP NTAs is shown in the inset in Fig. 4E and presents a broken NiP nanotube. The morphology of CoAl-LDH on Ni foam was also characterized by FESEM (Fig. S2, Supporting Information), which demonstrate the network structure of CoAl-LDH nanosheets. Fig. 4C and F show the FESEM image of NiP@CoAl-LDH NTAs. Obviously, a layer of CoAl-LDH nanosheets are interconnected with each other and homogeneously coated on the surface of NiP nanotube. Besides, the nanotube structure is still well maintained after coating CoAl-LDH nanosheets. Such nanotube arrays structure can effectively facilitate the superior electrochemical performance of NiP@CoAl-LDH NTAs electrode. 3.2. Electrochemical performances To explore the electrochemical performances of the samples, cyclic voltammograms (CV) measurement was performed in the three-electrode cell in 2 M KOH aqueous solution. Fig. 5a shows the CV curves of NiP NTAs, CoAl-LDH and NiP@CoAl-LDH NTAs electrodes in the voltage range of 0-0.6 V at a scan rate of 5 mV s-1. The obvious redox peaks indicated the Faradic characteristics of the samples. For NiP NTAs electrode, a pair of redox peak at 0.45 and 0.35 V can be clearly observed, which can be attributed to the reversible reaction of Ni(OH)2/NiOOH. The Faradic reactions proceed as the following reactions [23, 25, 32]: Ni + 2OH−→Ni(OH)2 +2e− Ni(OH)2 + OH−↔NiOOH + H2O+e−

(7) (8)

For CoAl-LDH electrode, the redox peaks can be assigned to the reversible reactions of Co2+/Co3+ and Co3+/Co4+ according to the following reactions [33]: Co(OH)2 + OH− ↔ CoOOH + H2O + e− CoOOH + OH− ↔ CoO2 + H2O + e−

(9) (10)

Compared with NiP NTAs and CoAl-LDH electrode, the redox peaks of NiP@CoAl-LDH NTAs 10

electrode

were

significantly

enhanced,

indicating

the

better

electrochemical

performance

of

NiP@CoAl-LDH NTAs electrode. Fig. 5b shows the CV curves of NiP@CoAl-LDH NTAs electrode at different scan rates from 5 mV s-1 to 50 mV s-1. A linear relationship between anodic peak current and square root of scan rate (Fig. S3, Supporting Information) suggests the good reversibility of the obtained NiP@CoAl-LDH NTAs electrode and confirms the Faradic process was controlled by hydroxyl ion [34]. Galvanostatic charge-discharge (GCD) measurement was also used to evaluate the specific capacity of the samples. Fig. 5c shows the GCD curves of NiP NTAs, CoAl-LDH and NiP@CoAl-LDH NTAs electrodes at a current density of 1 mA cm-2. Apparently, NiP@CoAl-LDH NTAs electrode possesses the longest discharge time, suggesting the highest specific capacity among the samples. Moreover, nonlinear discharge curves indicated the Faradic behavior of all the electrodes, matching well with the CV result. Fig. 5d shows the GCD curves of NiP@CoAl-LDH NTAs electrode at different current densities. According to eqn (1), the corresponding areal specific capacity (Ca) were calculated to be 0.67 C cm-2 (Cm of 556 C g-1) at 1 mA cm-2 with a retention of 74.1% at 20 mA cm-2, much higher than CoAl-LDH (Ca of 0.11 C cm-2, Cm of 367 C g-1 and retention of 67.2%) and NiP NTAs (Ca of 0.28 C cm-2, Cm of 316.22 C g-1 and retention of 73.8%), as shown in Fig. 5e and f. Furthermore, the specific capacity and capacity retention of NiP@CoAl-LDH NTAs electrode in this study are much higher than most of LDHs-based core-shell structure reported before, such as CoAl-LDH@PEDOT core-shell nanoplatelet array (504 C g-1 at 1A g-1, capacity retention of 63.09 %) [35], Fe3O4@C@NiAl-LDHs core-shell nanospheres (268.7 C g-1 at 1A g-1, capacity retention of 52.3 %) [36], and SiO2@NiAl-LDHs core-shell microspheres (203 C g-1 at 2 A g-1, capacity retention of 48.1 %) [37]. Such high specific capacity and capacity retention also prove the superior advantages of NiP@CoAl-LDH NTAs electrode. To further evaluate the electrochemical property of the samples, the electrochemical impedance spectroscopy (EIS) measurement was performed in 2 M KOH solution. Fig. 4S (In Supporting Information) 11

presents the Nyquist plots of CoAl-LDH, NiP NTAs and NiP@CoAl-LDH NTAs at a potential of 0.2 V in the range of 100 kHz-0.01 Hz. All the plots are composed of a semicircle at the high-frequency region, a straight line at the low-frequency region and an intercept of the real axis, which attribute to the charge transfer resistance (Rct), the diffusive resistance (W) and the internal resistance (Re), respectively. Obviously, the NiP@CoAl-LDH NTAs possesses a lower charge transfer resistance (Rct: 0.70 Ω) than CoAl-LDH (Rct: 1.61 Ω), suggesting the faster electron transport for NiP@CoAl-LDH NTAs electrode. Such enhanced electron transport can be assigned to the high conductivity of NiP NTAs (Rct: 0.52 Ω). In addition, the line slope of the NiP@CoAl-LDH NTAs is much higher than that of CoAl-LDH, indicating the lower diffusive resistance of the NiP@CoAl-LDH NTAs resulting from the core-shell nanotube structure. Thus, the results mentioned above illustrate that the combination of fast electron transport and ion diffusion rate is responsible for the outstanding electrochemical property of NiP@CoAl-LDH NTAs. Based on the above results, NiP@CoAl-LDH NTAs electrode possesses a high specific capacity, an excellent rate capability and a fast electron transport and ion diffusion rate, and this can be ascribed to the following advantages: (i) NiP and CoAl-LDH are good Faradic electrode materials; (ii) the unique nanotube arrays can provide a short diffusion path for electrolyte ions, and the inner core of nanotube as a framework can facilitate a fast transport for electron at high power density; (iii) the NiP@CoAl-LDH NTAs directly grown onto the conductive substrate can provide excellent electrical connection with current collectors, enable each NiP@CoAl-LDH nanotube to effectively participate in Faraday reactions and result in a high utilization rate for electrode materials. These favorable features will eventually lead to enormous enhancement of electrochemical properties. To further evaluate the practical application of NiP@CoAl-LDH NTAs electrode, an asymmetric supercapacitor (ASC) device was assembled using NiP@CoAl-LDH NTAs as the positive electrode and AC on Ni foam as the negative electrode in 2 M KOH aqueous solution (denoted as NiP@CoAl-LDH 12

NTAs//AC), as shown in Fig. 6a. To determine the voltage window of ASC device, CV curves of the NiP@CoAl-LDH NTAs//AC at different voltage windows were measured at a scan rate of 50 mV s-1 and shown in Fig. 6b. As expected, the voltage window of ASC device can be extended to 1.6 V. Fig. 6c shows the CV curves of ASC device at different scan rates, suggesting double contribution of Faradaic capacity and electric double-layer capacitance. GCD test was also performed to further evaluate the electrochemical performance of ASC device at different current densities (Fig. 6d). And the corresponding mass specific capacity were calculated to be 156.1 C g-1 at a current density of 2 mA cm-2 and 94.2 C g-1 at a high current density of 20 mA cm-2, as shown in Fig. 6e. Based on these Cm values, the energy densities of ASC device were further calculated to be 37.18 Wh kg-1 at a power density of 0.45 kW kg-1 and 23.27 Wh kg-1 at 4.68 kW kg-1 (result shown in Fig. 6f), much higher than the previous reported LDH-based ASC devices, such as NixCo1-xLDH-ZTO//AC (23.7 Wh kg-1) [38], [email protected](OH)2//CMK-3 (31.2 Wh kg-1) [19] and CoAl-LDH//AC (25.1 Wh kg-1) [39]. With such a high energy density, the obtained ASC device demonstrated the ability to run a red light-emitting diode (LED), as shown in the inset of Fig. 6f. Cycling performance of NiP@CoAl-LDH NTAs//AC device was recorded on a LAND-CT2001A system by continuous charge-discharge process. Fig. 7a shows the cycle life of NiP@CoAl-LDH NTAs//AC device over 4000 cycles at a current density of 6 mA cm-2. The increase of specific capacity for the first 1000 cycles can be assigned to the activated process of the electrode materials. As expected, the overall capacity retains about 95.50 % of the initial specific capacity. In addition, the core-shell nanotube structure of NiP@CoAl-LDH NTAs is well maintained after cycles as shown in the inset of Fig. 7a, which also prove the long cycle life of ACS device. Such excellent cycle stability may be attributed to the strong connection between nanotubes and result in much less vulnerable of electrode material to dissolution. To further evaluate the influence of cycle process to the structure of ACS device, EIS spectra was measured in the range of 100 kHz-0.01 Hz. Fig. 7b shows the Nyquist plots of ACS device before and after cycles. The 13

corresponding charge transfer resistance values (Rct: semicircle in the high-frequency region) were calculated to be 1.03 and 0.93 Ω, and the internal resistance values (Re: intercept of the real axis) were 1.03 and 1.15 Ω, respectively. Furthermore, both Nyquist plots also feature a similar diffusive resistance (W: sloped line in the low-frequency region). These results indicate the structural stability of NiP@CoAl-LDH NTAs in cycle process and further demonstrates that the ACS device have a relatively good cycling performance.

4.

Conclusions In summary, we have successfully synthesized a novel NiP@CoAl-LDH NTAs for supercapacitors.

Compared with NiP NTAs and CoAl-LDH electrodes, the obtained NiP@CoAl-LDH NTAs electrode possesses a higher specific capacity and excellent rate capacity due to the aligned core-shell nanotube structure and the Faradic characteristics of CoAl-LDH and NiP. Moreover, the ACS device prepared by taking NiP@CoAl-LDH NTAs as positive electrode and AC as negative electrode can achieve a mass specific capacity of 156.1 C g-1 at 2 mA cm-2 and a maximum energy density of 37.18 Wh kg-1 at a power density of 0.45 kW kg-1, as well as long-term cycle stability (95.50 % retention at 6 mA cm-2 after 4000 cycles). Such superior electrochemical behaviors are expected to be a promising candidate for the application of supercapacitors.

Acknowledgements The authors are grateful to the testing support from the Analysis and Testing Center (Xiamen University)

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Figures Fig. 1 Schematic illustration for the fabrication of NiP@CoAl-LDH NTAs.

Fig. 1

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Fig. 2 (a) XRD pattern of ZnO NRAs and NiP@CoAl-LDH NTAs; (b) XPS Spectra of Ni 2p in NiP NTAs; (c) XPS Spectra of P 2p in NiP NTAs; (d) TEM and HRTEM (inset) images of NiP NTAs.

Fig. 2

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Fig. 3 (a) EDS analysis of NiP@CoAl-LDH NTAs; (b, c and e) TEM and HRTEM images of NiP@CoAlLDH NTAs; (d) SAED pattern of shell.

Fig. 3

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Fig. 4 FESEM images of ZnO NRAs (A, D), NiP NTAs (B, E) and NiP@CoAl-LDH NTAs (C, F) with different magnifications.

Fig. 4

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Fig. 5 (a) CV curves of NiP NTAs, CoAl-LDH and NiP@CoAl-LDH NTAs electrodes at a scan rate of 5 mV s-1; (b) CV curves of NiP@CoAl-LDH NTAs electrode at different scan rates; (c) GCD curves of NiP NTAs, CoAl-LDH and NiP@CoAl-LDH NTAs electrodes at a current density of 1 mA cm-2; (d) GCD curves of NiP@CoAl-LDH NTAs electrode at different current densities; (e) Areal specific capacity of NiP NTAs, CoAl-LDH and NiP@CoAl-LDH NTAs electrodes at different current densities; (f) Mass specific capacity of NiP NTAs, CoAl-LDH and NiP@CoAl-LDH NTAs electrodes at different current densities.

Fig. 5

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Fig. 6 (a) Schematic illustration of the ACS device; (b) CV curves of ACS device at a scan rate of 50 mV s -1 with different voltage windows; (c) CV curves of ACS device at different scan rates; (d) GCD curves of ACS device at different current densities; (e) Mass specific capacity of ACS device at different current densities; (f) Ragone plots (Energy density vs Power density) of ACS device and digital photograph (inset) of the lighten-up LED driving from two ACS device.

Fig. 6

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Fig. 7 (a) Long-term cycle life of ACS device at a current density of 6 mA cm-2 and FESEM image (inset) of NiP@CoAl-LDH NTAs after cycles; (b) Nyquist plots of ACS device before and after cycles.

Fig. 7

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