Construction of Ni-Co-Mn layered double hydroxide nanoflakes assembled hollow nanocages from bimetallic imidazolate frameworks for supercapacitors

Accepted Manuscript Title: Construction of Ni-Co-Mn layered double hydroxide nanoflakes assembled hollow nanocages from bimetallic imidazolate frameworks for supercapacitors Authors: Xianli Zheng, Xue Han, Xiaoxian Zhao, Jian Qi, Qingxiang Ma, Kai Tao, Lei Han PII: DOI: Reference:

S0025-5408(18)30884-5 https://doi.org/10.1016/j.materresbull.2018.06.005 MRB 10043

To appear in:

MRB

Received date: Revised date: Accepted date:

25-3-2018 23-5-2018 3-6-2018

Please cite this article as: Zheng X, Han X, Zhao X, Qi J, Ma Q, Tao K, Han L, Construction of Ni-Co-Mn layered double hydroxide nanoflakes assembled hollow nanocages from bimetallic imidazolate frameworks for supercapacitors, Materials Research Bulletin (2018), https://doi.org/10.1016/j.materresbull.2018.06.005 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.

Construction of Ni-Co-Mn layered double hydroxide nanoflakes assembled hollow nanocages from bimetallic imidazolate frameworks for supercapacitors Xianli Zheng,‡a Xue Han,‡a Xiaoxian Zhao,c Jian Qi,d,e Qingxiang Ma,b Kai Tao*a, b

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and Lei Hana a. School of Materials Science & Chemical Engineering, Ningbo University, Ningbo,

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Zhejiang 315211, China. E-mail: [email protected] (K. Tao).

b. State Key Laboratory of High-efficiency Coal Utilization and Green Chemical

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Engineering, Ningxia University, Yinchuan 750021, P.R. China.

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c. Department of Physical Chemistry School of Metallurgical and Ecological

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Haidian District, Beijing 100083, China.

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Engineering, University of Science & Technology Beijing No. 30, Xueyuan Road,

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d. State Key Laboratory of Biochemical Engineering, Institute of Process Engineering,

China.

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Chinese Academy of Sciences, 1 North 2nd Street, Zhongguancun, Beijing 100190,

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e. University of Chinese Academy of Sciences, No. 19A Yuquan Road, Beijing 100049, P.R. China

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‡ These authors contributed equally to this study.

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Graphic Abstract

Ni-Co-Mn LDH nanoflakes assembled hollow nanocage has been successfully

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fabricated by a facile bimetallic Co-Mn ZIF templating strategy.

Highlights

Bimetallic Co-Mn ZIF is successfully prepared.

2. N-Co-Mn LDH

nanoflakes assembled hollow nanocage

method.

is constructed by a MOF templating

LDH exhibits excellent pseudocapacitive performance.

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3. The N-Co-Mn

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1.

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Abstract

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Layered double hydroxides (LDHs) with 3-dimentional (3D) hollow nanoarchitetures

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are highly desirable for energy related applications. In this study, Ni-Co-Mn LDH nanoflakes assembled hollow nanocages are constructed from bimetallic imidazolate

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framworks precursors. Benefiting from the 3D hierarchical porous strcuture and

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composition, the as-synthesized ternary Ni-Co-Mn LDH hollow nanocage, as a electrode for supercapacitor, demonstrates a remarkable electrochemical performance with a high specific capacitance of 2012.5 F g-1 at 1 A g-1 and a good rate capacity of

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75.0 % retention at 10 A g-1, significantly outperforming binary Ni-Co LDH (1266.2 F g-1 at 1 A g-1, 41.8% retention at 10 A g-1). This work demonstrates the construction of 3D hollow nanostructured LDH with multicomponent compositions for the first

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time, and can be simply extended to construct other hollow structured electrode materials for energy storage devices.

Keywords: LDH; Ni-Co-Mn; metal-organic frameworks; supercapacitor

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1. Introduction In the past decades, energy crisis and deteriorating environment have forced us

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to exploit sustainable energy, and relevant energy conversion and storage

devices. Among various energy storage devices, supercapacitors (SCs) have

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drawn considerable attention because of their high power density, long cycle

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life and good safety.[1, 2] Pseudocapacitive materials (metal oxides, metal

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sulfides, conductive polymers etc.) can achieve much higher specific

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capacitances compared with electric double-layer capacitive carbon materials

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owing to their rich Faradic redox reactions.[1, 3-6] Recently, layered double hydroxide (LDH), as a new type of pseudocapacitive materials, has been

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attracting intensive research attention due to its high redox activity, tailorable

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composition, low cost and environmental friendliness features.[7-9] However, the widespread applications of LDH as electrode materials for SCs are

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hampered by their intrinsically poor electric conductivity and strong tendency to stack together.[10] In this regard, rational construction of various 3-dimentional (3D) nanostructures for LDH is a popular strategy to tackle these problems. Hollow structured LDH is of particular interest for SCs, because of the large surface area and substantial active sites exposed, facilitated charge 3

transfer and reduced aggregation.[11] Therefore, numerous methods have been developed to fabricate hollow nanostructured LDH. Typically, the 3D hollow structures can be prepared by employing a sacrificial template. For instance, Ni-Co LDH and Ni-Mn LDH hollow microspheres were synthesized by

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co-precipitation method using SiO2 microspheres as templates, and they exhibited good electrochemical capacitive properties.[12] But, the hard

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template removal process is usually time-consuming and also causes severe

environmental pollution. Besides, current works are mainly focused on the

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synthesis of hollow LDH spheres. The synthesis of non-spherical spherical

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LDH is much more challenging and needs much more delicate control of

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experiment parameters.[13] On the other hand, the electrochemical capacitance

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of binary LDH/metal oxide could be improved by the doping hetero-metal due

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to the widened interlayer space and alleviated mechanical stress during charge-discharge process.[14-16] For example, 1D mesoporous ternary Co–Ni–

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Mn oxide nanowires with multiple valences and large electroactive surface area

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showed good pesudocapacitance and excellent cycling stability.[17] It was reported that ternary NiCoAl-LDH exhibited higher specific capacitance compared with binary Ni–Al LDH or Co–Al LDH.[18] Thus, rational design of

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ternary LDH, such as Ni-Co-Mn LDH with well-defined 3D hollow structure and multicomponent compositions

may help boosting electrochemical

performance, but still remains challenging.

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Metal-organic frameworks (MOFs) consists of metal nodes and organic ligands, with permanent porosity and tailorable functionalities, have promising applications in gas separation,[19, 20] sensor,[21] catalysis,[22] drug delivery,[23] and so on. MOFs, in particular, zeolitic imidazolate frameworks

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(ZIFs, a sub-class of MOFs), have recently emerged as ideal precursors or sacrificial templates for preparing numerous functional porous materials owing

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to their highly porous and ordered structure, as well as tunable composition.[24] Very recently, Jiang et al.[25] have successfully synthesized a hollow Ni-Co

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LDH nano-polyhedra using ZIF-67 nanocrystal as the template in an ethanol

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solution of nickel nitrate. Due to the novel hierarchical structure, the

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as-synthesized Ni-Co LDH exhibited superior electrochemical property. A

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similar method was used to prepare a series of Ni-Co LDHs.[8] It was found

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that the amount of Ni(NO3)2 had a pronounced influence on the morphology of Ni-Co LDHs and corresponding electrochemical properties. Ni-Co LDHs were

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also synthesized from ZIF-67 templates in water-ethanol solution of Ni(NO3)2,

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and the water played a crucial role in the formation of hollow Ni–Co LDH microstructure.[26] A Co-Co LDH/graphene was synthesized by deposition of ZIF-67 on graphene with additional etching by Co(NO3)2, displaying a high

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specific capacitance of 1205 F g-1 at a current density of 1 A g-1.[27] Notwithstanding these progresses on design of hollow LDHs using MOFs as templates, there is no report on the synthesis of ternary LDH, such as Ni-Co-Mn LDH from MOFs. 5

Inspired by the above works, herein, we first report the synthesis of ternary Ni-Co-Mn LDH nanoflakes assembled hollow nanocage by a facile method using a bimetallic Co-Mn ZIF as precursor and sacrificial template. As expected, the as-prepared ternary Ni-Co-Mn hollow LDH nanocage exhibits

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excellent electrochemical capacitive performance, outperforming binary Ni-Co LDH. The excellent electrochemical performance makes Ni-Co-Mn LDH

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promising electrode materials for various electrochemical energy conversion

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and storage devices.

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2. Experimental

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2.1 Chemicals and Reagents

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Cobalt nitrate hexahydrate (Co(NO3)2•6H2O), manganese nitrate tetrahydrate

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(Mn(NO3)2•4H2O), nickel nitrate hexahydrate (Ni(NO3)2•6H2O), 2-methylimidazole (Hmim) and ethanol were supplied by Sinopharm Chemical Reagent Co., Ltd. All the

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purification.

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chemicals and solvents were of analytical grade, and were used without further

2.2 Preparation of bimetallic Co-Mn-ZIF

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Typically, 0.126 g of Mn(NO3)2·4H2O and 0.291 g of Co(NO3)2·6H2O were dissolved in 3 mL of H2O to form solution A. 5.502 g of Hmim was dissolved in 30 mL of H2O to form solution B. Then, solution A and B were combined, and the mixture was vigorously stirred at room temperature for 24 h. Finally, dark purple Co-Mn-ZIF was

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collected by centrifugation, washing with H2O, and drying at 70 oC. Co-ZIF (ZIF-67) was also synthesized by using the same method as Co-Mn-ZIF without addition of Mn(NO3)2·4H2O.

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2.3 Synthesis of Ni-Co-Mn LDH hollow nanocage Ni-Co-Mn LDH was synthesized using Co-Mn-ZIF as precursor and template. In a

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typical procedure, 80 mg of as-prepared Co-Mn-ZIF were dispersed in 25 mL of

absolute ethanol. To which, 0.09 g of Ni(NO3)•6H2O was added. The mixture was refluxed at 90 oC for 1 h. After cooling to room temperature, the samples were o

C. For

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collected by centrifugation, washing with ethanol, and drying at 70

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comparison, the Ni-Co LDH was also synthesized under the same conditions except

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2.4 Characterization

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using Co-ZIF as the template and precursor.

X-ray diffraction (XRD) patterns were acquired on a Bruker AXS D8 Advance

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diffractometer using a Cu Ka radiation (λ = 1.5406 Å). Fourier transformation infrared (FT-IR) spectra were collected on a NICOLET-5700 FTIR

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spectrophotometer. The morphology and composition of the sample were characterized by a field emission scanning electron microscope (FESEM,

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Hitachi S-4800) equipped with an energy dispersive spectroscopy (EDS), and a transmission electron microscope (TEM, FEI Tecnai TF20). N 2 sorption isotherm was measured on a Micrometrics ASAP-2020M adsorption apparatus. The specific surface area was determined by the multiple Brunauer–Emmett– 7

Teller (BET) method. The pore size distribution was obtained from the adsorption branch of the isotherm using Barrett–Joyner–Halenda (BJH) method. The surface chemical states of elements of sample were analyzed by X-ray photoelectron spectroscopy (XPS, ESCA-Lab-200i-XL).

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2.5 Electrochemical measurements

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The electrochemical measurements were carried out in a three-electrode cell using a 1 M KOH solution as the electrolyte. The MOF derived LDH, platinum foil electrode, and saturated calomel electrode (SCE) were used as working,

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counter, and reference electrode, respectively. The working electrode was

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prepared by mixing LDH (80 wt.%), acetylene black (10 wt.%) and

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polyvinylidene difluoride (PVDF, 10 wt.%) with ethanol to form a paste, which was coated onto a nickel foam collector (1 × 1 cm). The resulting electrode was

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dried at 65 oC for 12 h and pressed under a pressure of 10 MPa. The loading of

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active material on Ni foam was around 1.5 mg. The electrochemical performance of the electrode was evaluated by cyclic voltammetry (CV), charge–discharge

(GCD)

spectroscopy

(EIS)

CHI660E

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galvanostatic

using

a

and

electrochemical

(Chenhua,

impedance

Shanghai,

China)

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electrochemical workstation. The specific capacitance can be calculated from the discharge curve using following equation: C=

I×∆t m×∆V

(1)

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where I (A) is the discharge current, Δt (s) is the discharge time, m (g) is the mass of active material and Δ V (V) is the potential range of discharge. 3. Results and discussion The synthesis of Ni-Co-Mn LDH nanoflakes assembled hollow nanocage is

Co-Mn-ZIF

polyhedron

was

prepared

by

mixing

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schematically demonstrated in Scheme 1. In the first step, bimetallic Mn(NO3)2·4H2O,

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Co(NO3)2·6H2O and Hmim in H2O at room temperature for 24 h. In the second

step, the Co-Mn-ZIF was dispersed in an ethanol solution of Ni(NO3)2·6H2O,

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and the mixture was refluxed at 90 oC for 1 h. During this process, Co-Mn-ZIF

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would gradually etched by the protons generated through the hydrolysis of Ni 2+

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ions to release cobalt and manganese ions, and Ni-Co-Mn LDH was formed by

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the co-precipitation of Ni, Co and Mn ions.[25, 28] As the reaction continued,

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the Co-Mn-ZIF template was completely dissolved and the Ni-Co-Mn LDH

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nanoflakes assembled hollow nanocage was formed.

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Scheme1. Schematic illustration for the synthesis of Ni-Co-Mn LDH nanoflakes assembled hollow nanocage. The crystalline structure and phase of the samples was analysed by XRD. As shown in Figure 1a, the XRD pattern of Co-Mn-ZIF matches well with that of simulated ZIF-67, and is also consistent with that of as-prepared ZIF-67 (Figure 9

S1), indicating that bimetallic Co-Mn-ZIF with high purity has been successfully synthesized by a facile aqueous route. For Ni-Co-Mn LDH nanocage, all the diffraction peaks at 11.9, 24.0, 34.3, 36.6 and 60.3 o, can be indexed to (003), (006), (012), (104) and (110) planes of hydrotalcite-like LDH

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phase.[29] No diffraction peaks corresponding to ZIF-67 could be detected after Co-Mn-ZIF is reacted with Ni(NO3)2, suggesting the full transformation to

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LDH phase. The functional groups in the samples were characterized by FTIR.

The spectrum of Co-Mn-ZIF is similar to that of ZIF-67 reported

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previously.[29] The peaks observed 600-1600 are characteristics of stretching

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and bending modes of imidazole ring, and the intense peak at 426 cm-1 is

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attributed to Co-N stretching mode.[2] For Ni-Co-Mn LDH sample, all of the

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characteristic peaks for Co-Mn-ZIF disappear after reacting with Ni(NO3)2. The

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peak at 1624 cm−1 is related to the interlayer water.[8] The sharp peak at 1383 cm-1 is ascribed to N-O stretching vibration of NO3-. The bands in 500-800 cm-1

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are indexed to M-O and M-OH bending vibration modes.[27] The FTIR results

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are consistent with the XRD results, further confirming that the LDH is

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successfully prepared.

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Figure 1. (a) XRD patterns of Co-Mn-ZIF and Ni-Co-Mn LDH; (b) FTIR spectra of Co-Mn-ZIF and Ni-Co-Mn LDH.

The surface morphology of Co-Mn-ZIF template and its derived Ni-Co-Mn

N

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LDH was examined by SEM and TEM observations. The typical SEM images

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of Co-Mn-ZIF and Ni-Co-Mn LDH are shown in Figure 2. The bimetallic

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Co-Mn-ZIF synthesized in aqueous phase displays a regular polyhedral nanocage shape (Figure 2a), which is analogous to that of ZIF-67 (Figure S2a,

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b). A magnified SEM image in Figure 2b suggests that the surface of the

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polyhedral nanocage is fair smooth. After reaction with Ni(NO3)2 under refluxing, the overall polyhedral nanocage shape of Co-Mn-ZIF is retained (Figure 2c). In

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contrast to the smooth surface of Co-Mn-ZIF, the surface of Ni-Co-Mn LDH nanocage is rougher and is assembled by several nanoflakes. The nanoflakes

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are crosslinked and interconnected to form a hierarchical porous structure. The Ni-Co-Mn LDH has hollow interior, as can be clearly seen from a broken nanocage (Figure 2d). EDS result (Figure S3) confirms the presence of Ni, Co, Mn and O in the Ni-Co-Mn LDH nanocage sample, and the molar ratio of Ni:Co:Mn is 2.3:1.6:1. Moreover, Ni, Co and Mn are uniformly distributed 11

throughout the Ni-Co-Mn LDH nanocage, as revealed by elemental mapping in

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A

N

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Figure 2e.

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Figure 2. (a, b) SEM images of Co-Mn-ZIF; (c, d) SEM images of Ni-Co-Mn LDH

nanocage.

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hollow nanocage; (e) SEM elemental mapping analysis of Ni-Co-Mn LDH hollow

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The hollow nanocage structure assembled by nanoflakes is further confirmed

by TEM image (Figure 3a). The nanoflakes with an average thickness of ~5 nm

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are interconnected to form a hierarchical porous structure (Figure 3b), which is in accordance with SEM results. The high-resolution electron microscopy (HRTEM) image in Figure 3c shows well-resolved lattice fringes with interplanar spacings of 0.26 and 0.24 nm, which correspond to the (012) and (104) planes of LDH, respectively,[29] and the result matches well with the 12

XRD results. Furthermore, the polycrystalline feature of the Ni-Co-Mn LDH hollow nanocage is revealed by the selected-area electron diffraction (SAED) pattern

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(inset of Figure 3c).

Figure 3. (a, b) TEM images of Ni-Co-Mn LDH; (c) HRTEM image of Ni-Co-Mn LDH (inset: SAED).

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The surface chemical oxidation states of Ni-Co-Mn LDH were analyzed by

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XPS. The survey spectrum (Figure 4a) indicates the existence of Ni, Co, Mn, O

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and C in the sample, which is consistent with EDS result. The presence of C is due to the exposure to air. The high-resolution Ni 2p spectrum (Figure 4b) is

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deconvoluted into two spin-orbit doublets at 873.1 and 855.3 eV with two

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additional satellites (Sat.), corresponding to Ni 2p1/2 and Ni 2p3/2 of Ni2+.[30] Figure 4c shows the high-resolution Co 2p spectrum, which can be spited into

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two prominent peaks at 796.1 eV (Co 2p1/2) and 780.9 eV Co 2p3/2), accompanied by two shakeup satellites. The spin–energy splitting of 15.2 eV

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and the presence of satellite peaks indicate the existence of both Co 2+ and Co3+ in Ni-Co-Mn LDH nanocage.[2, 31] The high-resolution Mn 2p spectrum in Figure 4d shows a broad band at 643. eV, which can be ascribed to Mn 2p3/2 (Mn4+).[32]

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Figure 4. XPS spectra of Ni-Co-Mn LDH: (a) survey spectrum; (b) Ni 2p; (c) Co 2p;

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(d) Mn 2p.

The specific surface area and porosity of Ni-Co-Mn LDH was characterized

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by N2 physisorption. The adsorption-desorption isotherm of Ni-Co-Mn LDH

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nanocage is presented in Figure 5a, from which typical type III isotherm with distinct hysteresis loop is observed, indicating the mesoporous structure in the Ni-Co-Mn LDH. The specific surface area calculated with the multiple

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Brunauer–Emmett–Teller (BET) method is as high as 53.9 m2 g-1. The large surface area can provide abundant active sites for Faradic reactions. The pore size distribution measured by Barrett–Joyner–Halenda (BJH) method is shown in Figure 5b. The sample possesses both mesopores and macropores, and the 14

pore size is centred ~29.3 nm. The presence of mesopores and macropore is beneficial for the fast ion transport, when Ni-Co-Mn LDH hollow nanocage is

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acted as electrode material for supercapacitor.

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Figure 5. (a) N2 adsorption–desorption isotherm of Ni-Co-Mn LDH; (b) pore size

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distribution of Ni-Co-Mn LDH.

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The electrochemical performance of the Ni-Co-Mn LDH hollow nanocage as the electrode material for supercapacior was studied by cyclic voltammetry

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(CV) and galvanostatic charge–discharge (GCD) in 1 M KOH using a

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three-electrode system. For comparison, binary Ni-Co LDH nanocage (Figure S2c, d) was prepared by the same method as that for synthesizing Ni-Co-Mn

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LDH, except that Co-Mn-ZIF was replaced by ZIF-67. The CV curves of Ni-Co-Mn LDH and Ni-Co LDH at a scan rate of 10 mV s-1 in the potential

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range between 0 and 0.5 V (vs SCE) are compared in Figure 6a. Each sample shows a pair of well-resolved redox peaks, indicating the pseudocapacitive behavior of the Ni-Co LDH and Ni-Co-Mn LDH electrodes.[3] Meanwhile, the area enclosed by the CV curve of Ni-Co-Mn LDH is larger than that of Ni-Co LDH, indicating that a higher specific capacitance is achieved on Ni-Co-Mn 15

LDH. The CV curves of the Ni-Co-Mn LDH at various scan rates ranging from 5 to 100 mV s-1 are presented in Figure 6b. With the scan rate increased from 5 to 50 mV s−1, the shape of the CV curve is well maintained, and the area enclosed

by

CV

curve

increases

significantly,

suggesting

the

good

plausibly ascribed to the following reversible reactions[16]:

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Ni(OH)2 +OH- ↔NiOOH+H2 O+e- (2) Co(OH)2 +OH- ↔CoOOH+H2 O+e- (3)

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MnOOH+OH- ↔MnO2 +H2 O+e- (6)

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CoOOH+OH- ↔CoO2 +H2 O+e- (4) Mn(OH)2 +OH- ↔MnOOH+H2 O+e- (5)

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electrochemical reversibility. The redox peaks in the CV curves can be

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Further increasing the scan rate to 100 mV s-1 results in polarization. The GCD

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curves of Ni-Co LDH and Ni-Co-Mn LDH at the current density of 1 A g-1 are presented in Figure 6c, which displays nonlinear curves with plateaus,

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indicating the pseudocapacitive behavior of the samples. Moreover, the

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discharge time of Ni-Co-Mn LDH electrode is longer than that of Ni-Co LDH electrode, suggesting a higher specific capacitance, as confirmed by CV results. The electrochemical performance of Ni-Co-Mn LDH and Ni-Co LDH were

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further investigated by the GCD measurements at different current densities ranging from 1 to 10 A g-1. On the basis of discharge curves (Figure 6d and Figure S4), the specific capacitances are estimated to 2012.5, 1955, 1903.1, 1779.7 and 1509.4 F g-1 at the current density of 1, 2, 3, 5 and 10 A g-1 , 16

respectively for the Ni-Co-Mn LDH electrode, whereas the specific capacitances of Ni-Co LDH are only 1266.2, 1170.6, 1082.3, 934.1 and 529.6 F g-1, at the current density of 1, 2, 3, 5 and 10 A g-1, respectively, as shown in Figure 6e. The Ni-Co-Mn LDH electrode exhibits much higher specific

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capacitance than that of Ni-Co LDH at the same current density. Besides, when current density is increased from 1 to 10 A g−1, 75 % and 41.8 % of the initial

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capacitance is retained for Ni-Co-Mn LDH and Ni-Co LDH, respectively, indicating that Ni-Co-Mn LDH delivers a better rate capability compared with

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Ni-Co LDH. The electrochemical impedance spectroscopy (EIS) tests were also

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carried, and the Nyquist plots of the electrodes with an equivalent circuit

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diagram are compared in Figure S5. The Ni-Co-Mn LDH exhibits a more

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vertical line in the low frequency compared with Ni-Co LDH, suggesting

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faster electrolyte diffusion within the electrode.[33] The semicircle in the high frequency represents the charge-transfer resistance (Rct), and the Rct of

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Ni-Co-Mn LDH (0.6 Ω) is smaller than that of Ni-Co LDH (1.35 Ω), implying

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higher charge transfer kinetics of Ni-Co-Mn-LDH.[34] The above results suggest that Ni-Co-Mn LDH exhibits superior electrochemical performance to Ni-Co LDH. The Ni-Co-Mn LDH also compares favourably with many LDH

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based supercapacitor reported in literatures (Table S1) in terms of specific capacitance, implying its promising applications in electrochemical energy storage devices. The nanoflakes assembled hollow nanocage with high surface area can provide rich electrochemical active sites for redox reactions. The 17

hierarchical mesoporous and macroporous structure facilitates ion diffusion and charge transportation. The ternary LDH possesses much higher conductivity and richer redox reactions than binary LDH. All these factors together contribute to the excellent electrochemical performance of Ni-Co-Mn LDH.

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Moreover, the cycling stability of the Ni-Co-Mn LDH was evaluated by 1000 cycles of GCD tests at the current density of 10 A g-1, as shown in Figure 6f.

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The specific capacitance declines gradually with the increase of cycles, and about 57.7 % of initial capacitance is retained after 1000 cycles. Further studies

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ED

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A

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as graphene) are being undertaken in our lab.

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to improve the cycling stability of Ni-Co-Mn LDH by introducing carbon (such

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Figure 6. (a) CV curves of Ni-Co-LDH and Ni-Co-Mn LDH at a scan rate of 10 mV s-1; (b) CV curves of Ni-Co-Mn LDH in the scan rate of 5-100 mV s-1; (c) GCD

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curves of Ni-Co-LDH and Ni-Co-Mn LDH at current density of 1 A g-1; (d) GCD curves of Ni-Co-Mn LDH in the current density of 1-10 A g-1; (e) specific capacitance versus current density of Ni-Co-LDH and Ni-Co-Mn LDH; (f) cyclic performance of Ni-Co-Mn LDH as a function of the cycle number.

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Conclusions In conclusion, Ni-Co-Mn LDH nanoflakes assembled hollow nanocage has been successfully fabricated by a facile bimetallic Co-Mn ZIF templated strategy. As a supercapacitor electrode, the as-prepared Ni-Co-Mn LDH exhibits high specific

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capacitance (2012.5 F g-1 at 1 A g-1) and good rate capacity (75.0 % retention at 10 A g-1), which is much higher than Ni-Co LDH (1266.2 F g-1 at 1 A g-1 and 41.8 %

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retention at 10 A g-1 ). The enhanced electrochemical performance is ascribed to the

large surface area, hierarchical porous structure and multicomponent compositions of

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Ni-Co-Mn LDH hollow nanocage, which can provide rich active sites and Faradic

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reactions, and rapid diffusion of electrons and electrolytes. This work may pave the

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way for the design and synthesis of high-performance electrode materials with

Acknowledgements

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controlled compositions and structures for energy storage devices.

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This work is financially supported by the NSF of China (51572272, 21471086,

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51572261), NSF of Ningbo (2017A610062), Foundation of State Key Laboratory of High-efficiency Utilization of Coal and Green Chemical Engineering (2016–09), the Science and Technology Department of Zhejiang

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Province (2017C33007), Youth Innovation Promotion Association of CAS (2017070) and the K.C. Wong Magna Fund in Ningbo University.

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