Construction of Hollow CobaltâNickel Phosphate Nanocages through

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The Construction of Hollow Cobalt-Nickel Phosphate Nanocages through a Controllable Etching Strategy for High Supercapacitor Performances Zhenyu Xiao, Yuxiang Bao, Zijian Li, Xudong Huai, Minghui Wang, Peng Liu, and Lei Wang ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.8b01627 • Publication Date (Web): 08 Jan 2019 Downloaded from http://pubs.acs.org on January 9, 2019

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ACS Applied Energy Materials

The Construction of Hollow Cobalt-Nickel Phosphate Nanocages through a Controllable Etching Strategy for High Supercapacitor Performances

Zhenyu Xiaoa, Yuxiang Baoa, Zijian Lib, Xudong Huaia, Minghui Wanga, Peng Liua, and Lei Wanga*

a

Key Laboratory of Eco-chemical Engineering, Taishan scholar advantage and characteristic

discipline team of Eco chemical process and technology, College of Chemistry and Molecular Engineering, Qingdao University of Science and Technology, Qingdao 266042, P. R. China. b

School of Materials Science and Engineering, Ocean University of China, Qingdao 266100, PR

China.

* Corresponding author email: [email protected] (L. Wang); Fax: +86 532 84023927; Tel: +86 532 84022681.

ACS Paragon Plus Environment

ACS Applied Energy Materials 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

Abstract Bi-metal phosphate, especially for those with hollow structure, have been recognized as promising materials for energy storage application. However, the researches of hollow bi-metal phosphates are quite rare, due to the hardship of controllable preparation process. In this work, through a shell-coating and then controllable etching process, a non-spherical hollow bi-metal (Co/Ni) phosphate nanocage (named ZIF-67-LDH-CNP-110) is successfully constructed for the first times, to the best of our knowledge. When utilized as an electrode material for supercapacitor application, the as-prepared nanocages exhibit a high specific capacitance of 1616 F∙g-1 at 1 A∙g-1 and excellent ratio capability of remaining 80.32% of initial capacitance at a high current density of 10 A∙g-1. In addition, the as-fabricated ZIF-67-LDH-CNP-110//AC hybrid supercapacitor presents a remarkable energy density of 33.29 Wh∙kg-1 at the power density of 0.15 kW∙kg-1.

Keywords: bimetallic phosphate, hollow structure, controllable etching, metal organic frameworks, supercapacitor


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Introduction Due to the intriguing structural features of high porosity, large surface area and low mass density, hollow micro-/nano-structures have attracted fast growing interests for energy storage and conversion systems

1-3.

The construction of hollow structures

can increase the contact area with electrolyte for maximum active material utilization, shorten the diffusion path of electronic for fast electrochemical kinetics and improve the structural robustness for better cycling performance. Nowadays, most of the constructed hollow structures are in spherical shape, which is due to the deficiency of non-spherical templates and thermodynamic favor during the ripening process. Compared with spherical hollow structures, non-spherical hollow structure with anisotropic morphologies may present unique properties, which have been highlighted recently. For example, the non-spherical hollow NiS box-in-box and Co3O4/NiCo2O4 nanocages, prepared by Lou and the co-workers, exhibit excellent supercapacitor performances

4,5.

Therefore, it is highly desirable, yet challenging, to fabricate the

non-spherical hollow structures with high-quality and controlled morphology for energy storage and conversion applications. In recent years, transition metal hydroxides/oxides/sulfides/phosphates have been widely applied as electrode materials for energy storage devices due to their high electrochemical activity and abundant resources. Compared to transition metal oxides or sulfides with tight packed molecular structure, transition metal phosphates with open framework can provide more active sites for improved capacitor performances 6-9.

Especially, bi-metallic phosphates possess higher conductivity and ameliorative



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supercapacitor properties than the single metal phosphates due to the synergistic effect of different metal species. For example, a cobalt-nickel phosphate with a Ni/Co ratio of 4:5 prepared by Pang and co-workers exhibits a specific capacitance of 1132.5 F∙g-1 at the current density of 1 A∙g-1, which is about 3 times higher than the single cobalt species 10. Similar reports have also been reported by Tang’s group 11 and Liu’s group

12,

and they concluded that the synergistic effect of nickel and cobalt species

may dramatically improve the specific capacitance and cycling stability of phosphate species. Though a few of related works have been reported, the controllable synthesis of bi-metallic phosphates with hollow structure and excellent energy storage application are quite rare. To the best of our knowledge, there are only one example of hollow NixCo3-x(PO4)2 reported for supercapacitor application

13,

which probably

due to the difficulties in achieving uniform hollow structures of metal phosphates and the less controllable size, phase, and morphology. Metal organic frameworks (MOFs), assembled by bridging organic links with highly dispersed metal or metal clusters, have been considered as an ideal template to construct porous nano-materials in energy storage and conversion applications, due to their controllable structure, high surface areas, and adjustable porosity. A large majority of MOFs derived transition metal-based oxides, sulfides, phosphides and layered double hydroxides have been prepared

14-22.

Attributed to the high surface

areas and rich channels inherited from MOF precursors, those MOF-derived nano-materials

commonly

show

enhanced

electrochemical

energy

storage

performances. Such as, the ZIF-67 derived hollow NiCo2O4 presents a high specific


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capacitance of 1055.3 F∙g-1 at 2.5 mA∙cm-2 23; and a Ni/Zn-BDC MOF derived hollow NiS2/ZnS spherical nano-composites exhibit an outstanding capacitance of 1198 F∙g-1 at 1 A∙g-1

24.

As another type of metal insoluble salt, there are only one example of

MOF-derived transition metal phosphates reported for supercapacitor application. The reported NixPyOz shows a high surface area of 142.24 m2∙g-1 and a improved specific capacitance of 1627 F∙g-1 at 1 A∙g-1 25. Herein, we reported a novel strategy for controllable synthesis of non-spherical hollow Co-Ni phosphate nanocages with hierarchical shell compositions using a metal-organic framework (ZIF-67) as template. This approach involves the formation of ZIF-67@Co-Ni layered double hydroxides (ZIF-67-LDH) and subsequent controllable etching process by Na3PO4 solution. By optimizing the synthesis process, the target hollow nanocages possess considerable active sites for energy storage, rich channels for electrolyte diffusion, and enough pore space for volume effect in charge/discharge process, as well as the synergistic effect of Ni/Co species, which present enhanced supercapacitor performances. Meanwhile, the possible formation mechanism of the hollow nanocage structure through etching was propounded. This work may provide a new direction of construction exquisite metal phosphate nano-structure via MOFs for high energy storage application.

Experimental Section Chemicals Cobalt nitrate hexahydrate (Co(NO3)2·6H2O, 99.9%, Energy Chemical), Nickel nitrate hexahydrate (Ni(NO3)2·6H2O, AR, Energy Chemica), 2-Methylimidazole



(C4H6N2, 98%, Energy Chemical), Sodium phosphate (Na3PO4, AR, Energy Chemical), All of the solvent used in this study were analytical grade. All materials in this study were obtained from commercial sources and used without further purification.

Synthesis of ZIF-67 Similar to the reported methods

26,

5.820 g Co(NO3)2 and 6.568 g 2-mIM were

dissolved in 500 mL methanol, respectively. Then the 2-mIM solution was quickly poured into the Co(NO3)2 solution under room temperature and the resultant mixed solution was aged for 24 h. The violet product was collected by centrifugation and then dried at 70 oC.

Synthesis of ZIF-67-LDH-CNP 160 mg of Ni(NO3)2 was dissolved in 50 mL EtOH, then 100 mg ZIF-67 was dispersed into the above solution. The mixture was stirred at room temperature for 0.5 h to obtain the ZIF-67@Co-Ni LDH. After that, 50 mL aqueous solution of 80 mg Na3PO4 was added and stirred at room temperature for 1 h. The solution was transferred to a 150 mL reaction vessel and kept at 110 oC for 15 h. After cooled to room temperature, the products were washed with EtOH several times and dried in 70 oC,

which were named as ZIF-67-LDH-CNP-110. As a control, a series of comparison

samples were prepared through similar procedure, such as removing the step of adding Ni(NO3)2 (named as ZIF-67-CP), removing the step of 0.5 h stirring (named as ZIF-67-CNP), removing the step of adding Na3PO4 (named as ZIF-67-LDH), the Ni(NO3)2 was replaced by Co(NO3)2 to obtained the ZIF-67@Co-Co LDH (named as


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ZIF-67-LDH-CP-110). controlling the temperature of solvothermal process (25 oC, 50 oC,

70 oC, 90 oC, 130 oC; named as ZIF-67-LDH-CNP-25, -50, -70, -90 and -130,

respectively).

Electrochemical measurements The electrochemical measurements were carried out using a CHI 760E electrochemical workstation and CT2001A under the ambient condition. In a three-electrode system, 6 M KOH was used as the aqueous electrolyte, a platinum wire (0.5 cm × 37 mm) as the counter electrode and an Ag/AgCl electrode (3 mol∙L-1) as the reference electrode. For the working electrode, a homogeneous slurry containing active materials, carbon black, and polytetra-fluorethylene (PTFE) with a weight ratio of 8:1:1 in ethanol was prepared and heated at 70 oC in vacuum for at least 12 hours. About 2.5 mg of the solidified mixture was then painted between two pieces of nickel foam (1 cm × 2 cm) and were pressed face to face under 1.0 MPa. The mass loading of the active material was accurately determined by the mass difference of the nickel foams before and after loading active materials. The electrochemical performances of the as-prepared electrode was examined by cyclic voltammetry (CV) and galvanostatic charge-discharge (GCD) measurements. Electrochemical impedance spectra were collected by applying a perturbation potential of 5 mV in the frequency range of 0.01 Hz to 106 Hz. The all-solid-state hybrid supercapacitor was fabricated using 1 mg active materials as the positive electrode, 5 mg activated carbon as the negative electrode, and PVA/KOH hydrogel polymer as the electrolyte. The activated carbon electrode



materials synthesized by mixing 18 mg activated carbon with polytetrafluoroethylene solution (8:1 for mass ratio). For PVA/KOH gel electrolyte, 2 g PVA was added into 80 ml H2O and got the cloudy solution, heating and stirring enough time to get the clear solution. Then 12 g KOH was added into the solution under stirring. The obtained gel solution was dried in the air finally.

Characterization Powder X-ray diffraction patterns of the as-prepared samples were collected on a Rigaku D-MAX2500/PC advance instrument with Cu-Kα radiation (λ = 1.5418 Å). The morphology and structure of the prepared samples were examined by electron microscopy (SEM, Zeiss merlin; TEM, FEI Tecnai G2 F20). The XPS spectrum were measured by Thermo Scientific 250xl. FTIR spectra were measured using Perkin Elmer Frontier FT-IR Spectrometer within the 4000-400 cm-1 region. The Brunauer-Emmett-Teller (BET) method was conducted to calculate the specific surface areas of samples by N2 adsorption-desorption measurement employing a surface area analyzer ASAP-2020.

Figure 1. Schematic illustration of the formation process of ZIF-67-LDH-CNP-110.

Results and Discussion The synthesis process of hollow Co-Ni phosphate nanocages are illustrated in Fig.


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1. The ZIF-67 templates were successfully prepared by mixed solution of Co(NO3)2 and 2-methylimidazoleare (2-mIM) and aged for 24 h

26.

Scanning electron

microscopy (Fig. S1a and 2a) and power X-ray diffraction (Fig. 3c) demonstrated the formation of pure ZIF-67 dodecahedron. After soaking the ZIF-67 polyhedron in Ni(NO3)2 solution for 0.5 h, the ZIF-67@Co-Ni LDH yolk-shelled structures were successfully fabricated (Fig. S1b)

4,27.

Then, through a controllable Na3PO4 solution

etching process via ZIF-67@Co-Ni LDH yolk-shelled structures as templates, the target hollow Co-Ni phosphate nanocage (ZIF-67-LDH-CNP-110) were finally achieved. As shown in Fig. S1c, the robust Co-Ni-LDH shell plays an important role to effectively alleviate the collapse of ZIF-67 nanocages in directly etching process, for which can dramatically slow down the etching rate of Na3PO4. Meanwhile, The SEM and TEM images for the sample obtained at etching 1 h, demonstrate that the etching is an outside to inside process (Fig. S2). In the etching process, the PO43anions can penetrate the Co-Ni-LDH shells and further destroy the framework of ZIF-67 to generate the free Co2+ ions. Then, the free Co2+ and Ni2+ ions will react with PO43- to form a thin layer of CoxNiy(PO4)z (CNP) and preferentially deposit on the reaction surfaces (Co-Ni-LDH shell), which finally results in the formation of hollow Co-Ni phosphate nanocages.



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Figure 2. The SEM images of ZIF-67 (a); and ZIF-67-LDH-CNP-110 nanocages ((b) and (c)). The TEM and EDS mapping of ZIF-67-LDH-CNP-110 nanocages (d). As shown in Fig. 2b, the as-obtained ZIF-67-LDH-CNP-110 remains the polyhedron structure of ZIF-67, and a hollow characteristic of these polyhedron can be observed from part of the broken nanocages. It is also observed that the shell of the nanocages are constructed by ultrathin and curved Co-Ni phosphate nanosheets with a thickness of a few nanometers (Fig. 2c and S3c). TEM images further confirm the hollow structure of ZIF-67-LDH-CNP-110 nanocages, which display a dark edges and bright centers (Fig. 2d-1 and S3a). From the enlarged view (Fig. S3b), the thickness of the shell was determined to be ~30 nm. The high-resolution TEM image (Fig. S3d) displays a weak lattice plane distance of 0.43 nm, corresponding to the (011) plane of Co3(PO4)2 (JCPDS: 13-0503). In addition, the corresponding selected-area

electron

diffraction

(SAED)

pattern

verified

that

the

ZIF-67-LDH-CNP-110 nanocages present a low crystallinity or even a amorphous characterristics (the insert in Fig. S3d), which is consistent with the XRD result. The


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elements distribution was investigated by element mapping analysis under a TEM observation. As shown in Fig. 2d, the Co, Ni and P elements are distributed homogeneously on the shell of nanocages. The EDX spectrum of the ZIF-67-LDH-CNP-110 nanocages present a Ni:Co ratio of around 6:4 (Fig. S5), which is similar to the ICP result of Ni:Co = 6.3:3.7. There is no N element is observed in the EDS spectra, which demonstrates that the framework of ZIF-67 is fully etched by PO43- ions. Such a desirable hollow structure can dramatically enhance the electrochemical active areas and promote migration rate of electrolyte ions through the ultrathin nanosheet spaces. The FTIR spectra of ZIF-67-LDH-CNP-110, ZIF-67 precursor and Ni3(PO4)2 via precipitation method are shown in Fig. 3a. Compared with ZIF-67 precursor, it can be found that the characteristic peaks of 2-mIM were disappeared in the spectrum of ZIF-67-LDH-CNP, indicating the 2-mIM- ligand was total substituted by PO43- units during the etching process. Moreover, the characteristic peaks of P-O stretching (~1037

cm-1)

and

O-P-O

bending

(~579

cm-1)

in

the

spectrum

of

ZIF-67-LDH-CNP-110, similar to Ni3(PO4)2, demonstrate the formation of related metal phosphate. Then the XPS analysis was performed to explore the elemental composition and chemical state of the prepared ZIF-67-LDH-CNP-110 nanocages (Fig. 3b). As shown in Fig. S6a, the fine-resolution Ni 2p spectrum shows four peaks at 856.6 eV, 862.3 eV, 874.3 eV, and 880.6 eV 25, respectively, corresponding to the 2p1/2 and 2p3/2 spin-orbit doublets and two shakeup satellites (sat.). Meanwhile, the spectrum of Co 2p can also be fitted with two spin-orbit doublets 2p1/2 at 782.0 eV



and 2p3/2 at 797.8 eV

26,27,

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and two satellites (sat.) at 786.2 eV and 803.4 eV,

respectively (Fig. S6b). For the P 2p spectrum, the binding energies appear at 133.4 eV consistent well with P6+ in the Ni3(PO4)2 28, as shown in Fig. S6c. The very weak N 1s high-resolution spectra (Fig. S6d) demonstrated the 2-mIM was almost completely removed, also supported by FTIR data. And, the C and O signal may come from adsorbed CO2

29.

The survey spectrum show that the Co, Ni, P and O

elements co-exist in the hollow nanocages, further confirmed the formation of bi-metal phosphate.

Figure

3.

(a)

The

FTIR

ZIF-67-LDH-CNP-110

and

ZIF-67-LDH-CNP-110.

(c)

spectra

Ni-PO4. The

of (b)

PXRD

ZIF-67, The

XPS

patterns

ZIF-67@Co-Ni survey of

LDH,

spectrum

ZIF-67,

of

Ni-PO4,

ZIF-67-LDH-CNP-110 and ZIF-67-CP. (d) The N2 adsorption and desorption isotherms and pore size distribution of ZIF-67-LDH-CNP-110 nanocages.


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Figure 4. The SEM of the ZIF-67-LDH-CNP-25 (a), -50 (b), -70 (c), -90 (d) -110 (e) and -130 (f) The

Power

X-ray

Diffraction

(PXRD)

of

ZIF-67

precursor

and

ZIF-67-LDH-CNP-110 were characterized to obtain the crystallographic phase and purity, as shown in Fig. 3c. It is obvious that the ZIF-67 have high crystalline degree that agree with the previously reported works

30.

However, ZIF-67-LDH-CNP-110

presents an obvious amorphous nature, which implies that the framework of ZIF-67 was completely destroyed and a new low crystalline phase was reconstructed during the etching process. The amorphous characteristic of ZIF-67-LDH-CNP-110 is beneficial for higher electrochemical performance for amorphous structure can provide larger ion accessible surface area and richer redox-active centers 31. Nitrogen adsorption-desorption isotherms at 77 K were measured to explore the porous structure and the surface area of the prepared ZIF-67-LDH-CNP-110. As shown in Fig. 3d, a H3-type hysteresis loop with a high surface area of 151.8 m2∙g-1 is observed that may attribute to a mesoporous structure constructed by ultrathin nanoplates,



which confirmed by the SEM, TEM and pore size distribution results (Fig. 2 and left inset of Fig. 3d). Those amorphous, high surface area and rich mesoporous pore features predict ZIF-67-LDH-CNP-110 will possess excellent electrochemical energy storage performances. The effect of etching temperature on the ZIF-67-LDH-CNP have been studied, in detail. Fig. 4 shows the SEM images of ZIF-67-LDH-CNP obtained at different etching temperature (25 oC, 50 oC, 70 oC, 90 oC, 110 oC and 130 oC). After 15 h etching at 25 oC, the CN-LDH shell were completely destroyed and the dodecahedron morphology of ZIF-67 were corroded to form a irregular ZIF-67 particles according to the PXRD result (Fig. S7). When the etching temperature rises to 50 oC, a hollow dodecahedron structure with a rough shell constructed by nanosheets were obtained. Similarly, the reaction temperature of 70 oC, 90 oC, and 110 oC can still produce the hollow Co/Ni phosphate nonocages, but the thickness of shell is gradually thinned with the increasing temperatures. For 110 oC, the hollow Co-Ni phosphate nanocages present the thinnest shell. As the temperature increased to 130 oC, a hybrid structure composed by nanoparticle and nanosheet was produced. These results demonstrate that the etching temperature between 50 oC and 110 oC is a proper temperature area in which Co/Ni phosphate nanosheets arrive a balance of deposition and dissolution on the shell of nanocages, which is crucial for the formation of hierarchical hollow Co/Ni phosphate nanocages. Fig. 5a shows the CV curves of the ZIF-67-LDH-CNP-110, ZIF-67-LDH (without the phosphate etching process), ZIF-67-CNP (without the Co-Ni-LDH shell


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constructing process) and ZIF-67-CP (without Ni(NO3)2·6H2O adding process) at the scan rate of 10 mV∙s-1 in 6 M KOH. It is observed that the ZIF-67-LDH-CNP-110 electrode delivers the largest area, indicating that the ZIF-67-LDH-CNP-110 electrode presents the best capacitance performance. The CV curves of ZIF-67-LDH-CNP-110 electrode at different scan rates are shown in Fig. S11e. A pair of redox peaks is observed which mainly attributes to the reaction of M2+ to M3+ (M = Co or Ni), and the reverse reduction processes can be expressed as in Equations S1. As the scan rate increases, the redox current increases which demonstrate that the charge storage mechanism was controlled by a semi-infinite diffusion process. Furthermore, these electrodes were tested by galvanostatic charge/discharge (GCD) measurements within a voltage range between -0.15 to 0.35 V (vs. Ag / AgCl) at different current densities from 1 to 10 A·g-1(Fig. S9 and Fig. 5b). As shown in Fig. 5b, the symmetric curves show that the ZIF-67-LDH-CNP-110 electrode possess a high charge/discharge coulombic efficiency, which are in good line with the CV curves (Fig. S11e). The specific capacitance of these electrodes were calculated via the Eq. S2 and the results were shown in Fig. 5c. The ZIF-67-LDH-CNP-110 electrode presents a specific capacitance of 1616 F∙g-1 and remains 80.32% of its capacitance (1298 F∙g-1) at a high current density of 10 A∙g-1, revealing an outstanding rate capability. For ZIF-67-CP electrode and ZIF-67-LDH-CP-110 electrode, only a low capacitance of 202 F∙g-1 and 300 F∙g-1 were obtained, respectively, which demonstrates the synergistic effect of Co and Ni elements plays an important role for improving the capacitance performance of Co-Ni phosphate

14,32,33.

However, the ZIF-67-CNP electrode also presents a low



specific capacitance of 640 F∙g-1 at 1 A∙g-1, which may be attributed to the destruction of hierarchical hollow nanocages that dramatically decreased the active surface area and ion migration channels (Fig. S1c and S8). Without the etching process of phosphate, a specific capacitance of 460 F∙g-1 were observed for ZIF-67-LDH, which demonstrated that the phosphate etching process will dramatically improve the conductivity of MOF precursors and boost the Faradaic processes across the interface 25.

Otherwise, the ZIF-67-LDH-CNP-110 electrode show an excellent cycling stability

of 72.46% or 34.48% retaining after 2000 or 5000 charge/discharge cycles at a current density of 2 A∙g-1, and steadily coulombic efficiency above 99.65% in 5000 cycles. In short, the outstanding electrochemical performances should owe to the novel hollow structure of ZIF-67-LDH-CNP-110 nanocages which provide more active sites, rich ion migration channels and enough pore volumes for reducing the volume effect, supported by BET and TEM measurements.

Figure 5. (a) The CV curves of the ZIF-67-LDH-CNP-110, ZIF-67-LDH, ZIF-67-CNP and ZIF-67-CP within the potential of -0.15-0.35 V at the scan rate of 10


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mV∙s-1. (b) The GCD curves of ZIF-67-LDH-CNP-110 at various charge/discharge current densities. (c) The capacity of the as-prepared ZIF-67-LDH-CNP-110, ZIF-67-LDH-CP-110, ZIF-67-LDH, ZIF-67-CNP and ZIF-67-CP as a function of current densities. (d) The cycling performance at the current density of 2 A∙g-1. Meanwhile, the mass ratio of ZIF-67 and Ni(NO3)2 has also been optimized to explore the influences of Co:Ni ratio for their electrochemical performances. The samples

obtained

with

different

mass

of

ZIF-67,

were

named

as

ZIF-67-LDH-CNP-110-m60, ZIF-67-LDH-CNP-110-m80, ZIF-67-LDH-CNP-110 and ZIF-67-LDH-CNP-110-m120, respectively. The ratios of Co and Ni elements have been carried out by Inductively coupled plasma (ICP) analysis, and the related data were shown in Table S1. The CV and GCD curves were also tested, and a capacitance of 966 F∙g-1, 1266 F∙g-1, 1616 F∙g-1 and 920 F∙g-1 were obtained for ZIF-67-LDH-CNP-110-m60, ZIF-67-LDH-CNP-110-m80, ZIF-67-LDH-CNP-110 and ZIF-67-LDH-CNP-110-m120 at 1 A∙g-1, respectively (Fig. S14 and S15). The results show that the Co:Ni = 1:1.08 of ZIF-67-LDH-CNP-110 is the best proportion of prepared bi-metal phosphate nanocages. The electrochemical impedance spectroscopy (EIS) from 0.01 Hz to 106 Hz was performed to evaluate the charge transfer and electrolyte diffusion in the electrode/electrolyte interface. As shown in Fig. S10, the Nyquist plots of the ZIF-67-LDH-CNP-110 electrode display a smaller diameter semicircle than the other electrodes, implying ZIF-67-LDH-CNP-110 possesses the lowest intrinsic resistance. In addition, the slope of the EIS curve for ZIF-67-LDH-CNP-110 electrode is much



higher than that of the other three electrodes, indicating that the fast diffusion/transportation process of the electrolyte ions and better Faradaic capacitive performance. The equivalent circuit of these electrodes were simulated by software Zview (the inset image in Fig. S10), and the related values were listed in Tab. S2.

Figure 6. (a) The CV curves, (b) GCD curves and (c) Cycling stability at 1 A∙g-1 of the ZIF-67-LDH-CNP-110//AC hybrid supercapacitor. (d) Ragon plot related to energy densities and power densities. Furthermore, the electrochemical performances of samples obtained at different etching temperature were also tested. As shown in Fig. S12, the GCD curves of the samples prepared at 25 oC, 50 oC, 70 oC, 90 oC, 110 oC and 130 oC are implemented, which present a specific capacitance of 572, 726, 836, 1416, 1616, 654 F∙g-1 at 1 A∙g-1, respectively. Along with the rise of etching temperature, the capacitance of the ZIF-67-LDH-CNP electrode increased firstly, and then decreased after reaching a maximum capacitance at 110 oC. The dramatically decreased capacitance of the


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sample obtained at 130 oC may cause by the destroyed hierarchical hollow nanocages structure. In order to evaluate the practical application of the optimized electrode material, a hybrid supercapacitor was fabricated using the ZIF-67-LDH-CNP-110 as positive electrode and active carbon (AC) as the negative electrode. The rectangular CV curves show the active carbon electrode presents typical EDLC behavior with a specific capacitance of 139.2 F∙g-1 at the current density of 0.8 A∙g-1 (Fig. S16). According to the charge balance theory, C-*ΔV-*m-= C+*ΔV+*m+34, an optimal mass ratio between ZIF-67-LDH-CNP-110 and AC was calculated to 1:5. The CV curves of fabricated hybrid supercapacitor are shown in Fig. 6a, which maintain similar shapes at different scan rates indicating a remarkable rate capability. Fig. 6b shows GCD curves of the prepared hybrid supercapacitor at different current densities within the

potential

window

of

0.0-1.5

V.

The

specific

capacitance

of

the

ZIF-67-LDH-CNP-110//AC hybrid supercapacitor are 116.67, 106.53, 91.33, 77.33, 68, 58.4, 51.2, 45.67 F∙g-1 at 0.1, 0.2, 0.5, 1, 2, 3, 4, 5 A∙g-1, respectively. The cycling stability of the ZIF-67-LDH-CNP-110//AC hybrid supercapacitor was further performed at a current density of 1 A∙g-1. As shown in Fig. 6c, the specific capacitance of the fabricated hybrid supercapacitor remained 67.24% of initial capacitance after 10000 cycles, implying an excellent cycling stability and cycling life. Fig. 6d shows the ragone plot of the ZIF-67-LDH-CNP-110//AC hybrid supercapacitor. A high energy density of 33.29 Wh∙kg-1 was attained at the power density of 0.15 kW∙kg-1, which is superior than most reported hybrid supercapacitor,



such as NiCo2S4//AC (25.5 Wh∙kg-1 at 0.334 kW∙kg-1) Wh∙kg-1at 0.175 kW∙kg-1)

36,

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35,

NiCo2O4//AC (14.7

NiCo2S4@NiO NWAs//AC (30.38 Wh∙kg-1 at 0.288

kW∙kg-1) 37, Ni-Co oxide//AC (7.4 Wh∙kg-1 at 1.9 kW∙kg-1) 38, NiCo2O4@MnO2//AC (35 Wh∙kg-1 at 0.163 kW∙kg-1)

39,

and so on. Furthermore, two solid-state flexible

devices in series can light a red LED (working votage: 3 V) (in set of Fig. 6c), which demonstrate the ZIF-67-LDH-CNP-110 is a kind of promising electrode material.

Conclusions In brief, through a shell-coating and then controllable etching process, a nonspherical hollow Co-Ni phosphate nanocages were firstly prepared via a metal-organic frameworks (ZIF-67) as template. Meanwhile, the shells of the hollow nanocages were constructed by ultrathin nanosheet, which increase the mounts of active sites, channels for electrolyte migration and pore volumes for reducing the volume effect. As well as the synergistic effect of Co and Ni species, the prepared ZIF-67-LDH-CNP-110 electrode presents high specific capacitance of 1616 F∙g-1 at 1 A∙g-1, enhanced rate capability and cycling stability. Furthermore, the fabricated ZIF-67-LDH-CNP-110//AC hybrid supercapacitor possesses excellent energy density of 33.29 Wh∙kg-1 at a power density of 0.15 kW∙kg-1, implying a great potential application in energy-storage devices. This work implies a new way to construct hollow metal phosphate for energy storage application. ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications


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website at DOI: xxxxxxx. SEM images of ZIF-67, ZIF-67@Co-Ni LDH, ZIF-67-CNP, ZIF-67 for 1 h etching and ZIF-67-LDH-CP-110; TEM images of ZIF-67@Co-Ni LDH, ZIF-67 for 1 h etching, ZIF-67-LDH-CNP-110 and ZIF-67-LDH-CP-110; HRTEM and SAED of ZIF-67-LDH-CNP-110; TEM mapping of ZIF-67-LDH-CP-110; The EDS and XPS spectra of ZIF-67-LDH-CNP-110; The XRD spectrum of ZIF-67-CP, Co3(PO4)2 and ZIF-67-LDH-CNP with different temperature (-25, -70 and -110); The N2 adsorption and desorption isotherms of the ZIF-67-CNP; The CV, GCD and Niquist plots of ZIF-67-LDH-CNP-110, ZIF-67-LDH, ZIF-67-CP, ZIF-67-CNP, ZIF-67-LDH-CNP obtained with different temperature, ZIF-67-LDH-CNP obtained with different mass ratios and active carbon electrode; ICP results of the prepared ZIF-67-LDH-CNP with different mass ratios. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] (Lei Wang) Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This work was supported by the National Natural Science Foundation of China (21805155,

51572136,

51772162,

21571112),

Taishan

Scholar

Foundation

(ts201511019), Shandong Provincial Natural Science Foundation （ZR2017BB038）.



Page 22 of 28

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TOC:


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