Rational Design of Sandwiched NiâCo Layered Double Hydroxides

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Rational Design of Sandwiched Ni-Co Layered Double Hydroxides Hollow nanocages/Graphene Derived from Metal-Organic Framework for Sustainable Energy Storage Xue Bai, Qi Liu, Zetong Lu, Jingyuan Liu, Rongrong Chen, Rumin Li, Dalei Song, Xiaoyan Jing, Peili Liu, and Jun Wang ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.7b01879 • Publication Date (Web): 01 Oct 2017 Downloaded from http://pubs.acs.org on October 4, 2017

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ACS Sustainable Chemistry & Engineering

Rational Design of Sandwiched Ni-Co Layered Double Hydroxides Hollow nanocages/Graphene Derived from Metal-Organic Framework for Sustainable Energy Storage Xue Bai, †,§ Qi Liu, *†,‡,§ Zetong Lu, ‖ Jingyuan Liu, †,§ Dalei †Key

†,‡,§ Rongrong

Chen, †,‡ Rumin Li,

Song, † Xiaoyan Jing, † Peili Liu, †,§ Jun Wang *†,‡

Laboratory of Superlight Material and Surface Technology, Ministry of

Education, ‡ Institute of Advanced Marine Material, §College of Materials Science and Chemical Engineering, Harbin Engineering University, No.145 Nantong Street, Harbin 150001, People’s Republic of China. E-mail: [email protected]:Fax:+86 451 8253 3062: Tel: +86 451 8253 3062 ‖Heilongjiang

University of Science and Technology, No.2468 Puyuan Street,

Harbin 150022, People’s Republic of China.

ACS Paragon Plus Environment


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Abstract In situ growth of Ni-Co layered double hydroxides on graphene nanosheets by virtue of metal-organic framework as sacrifice template is reported, which yields hollow nanocages uniformly deposited on graphene nanosheets. The strong impact of graphene amount on the electrochemical performance of Ni-Co layered double hydroxides is illustrated. Controlling the mass of graphene (15 mg) leads to a maximum specific capacitance of 1265 F g−1, high rate capability (50% capacitance retention after increasing current density ten times) and good cycling life (92.9% capacitance retention after 2000 circles). The combination of battery-type Ni-Co LDH hollow nanocages/graphene composite and active carbon allows for the excellent electrochemical performance measured in asymmetric device. In detail, the assembled asymmetric supercapacitor is able to deliver maximum specific capacitance 170.9 F g−1 in a potential window of 0-1.7 V, high energy density (68.0 Wh kg−1) as well as excellent power output (4759 W kg−1). These electrochemical performances, in combination with its facile fabrication, render hollow Ni-Co LDH/graphene composite as a promising electrode material in sustainable energy storage device. Keywords: sustainable ; supercapacitor; metal organic frameworks; layered double hydroxides ; graphene


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1 Introduction With the aim of reducing the dependence for ever-decreasing demand of fossil fuel, various energy storage devices are employed.

1-3

Supercapacitors, also called

ultrcapacitor, have attracted intense attention as sustainable energy storage devices by virtue of their outstanding properties, such as environmental friendly, advanced cycle stability, and, in particular power delivery.

4-6

Nevertheless, driven by the concern

over the relative low energy density, the improvement of traditional supercapacitor technology with high performance appears to be vital in developing supercapacitor. In this aspect, asymmetric supercapacitor, utilizing battery-type material as positive electrode and carbon material as negative electrode, can result in an improvement in power and energy density simultaneously. The development of high performance battery-type electrode has become an important factor for building such device. Therefore, the higher performance of battery-type material, the better properties of asymmetric supercapacitor could be obtained. Transition metal hydroxide, as an important part of battery-type materials, has been widely studied in asymmetric supercapacitor. Regarding metal hydroxide, more research attention has been attracted by layered double hydroxides. 7 Layered double hydroxides, are well-known anionic or hydrotalcite-like clays, whose structure is composed

of

positively-charged

compensating anions.

8,

9

brucite-like

layers

and

interlayer

charge

Considering the special layered structure and

anion-exchange stability, recently, layered double hydroxides (LDH) have been widely investigated in various fields, such as catalysis, separation and biotechnology. 10

Particularly, transition metal doped layered double hydroxide hybrids have been

conceived to be one of potential candidate in battery-type electrodes ascribed to their high redox activity, low cost and long cycle life.

11-13

However, LDH materials are



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still limited to the inherent low conductivity and severe aggregation, which greatly impedes the rate capability and energy density.

14

Rational design and fabrication of

LDH materials in nanostructure or incorporation of LDH with high conductive material to form hybrid composite, are supposed to be essential in improvement of electrochemical properties. Hollow micro/nanostructures with controllable size, shape and architecture have attracted intense attention ascribed to their prominent features of distinguishable interior voids, low density, large specific area, and reduced transport lengths for both mass and charge transport. 15 Until now, the researchers propose numerous methods to controllable synthesis hollow structure based on soft or hard template.

16-18

Rather

complex fabrication approaches are involved, which results in increasing the difficulty in building hollow structure. Therefore, seeking for facile method to construct hollow structure still remains a huge challenge. Very recently, many researchers have reported various hollow architectures based on metal-organic frame ZIF-67, such structure is feasible to convert to nano-hollow structures without sacrificing the main shape of polyhedron. Lou’s group reports a facile two-step diffusion-controlled strategy to generate hierarchical hollow prisms composed of nanosized CoS2 bubble-like subunits for lithium-ion batteries.

19

This group also

reports the synthesis of novel Co3O4/NiCo2O4 box-in-box nanocages (NCs) with different

shell

compositions,

electrocatalytic properties.

20

exhibiting

enhanced

pseudocapacitive

and

Such structure shows exceptional electrochemical

performance which may due to the exposure of high specific surface for faradic reactions. Unfortunately, a severe issue associated with low conductivity significantly affects the improvement of electrochemical properties. Overcoming the conductivity of material in a designed method may bring more possibilities in modulating the


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properties of functional nanostructures for electrodes. In this aspect, various approaches to confine battery-type material with many carbonaceous materials, have been used in energy storage applications, including carbon nanotubes, active carbon and graphene.

21-25

As an ideal matrix, graphene is

commonly used for growth of functional nano-materials. Specially, nano-composites made by graphene and battery-type materials, have been widely studied due to the synergistic effect arising from two materials.

26

On the one hand, battery-type

materials can provide many redox reaction sites which are the origin of high specific capacitance, on the other hand, graphene can effectively enhance the conductivity of battery-type mterials. Meanwhile, the anchoring of nanomaterials on graphene can effectively reduce the restacking of graphene nanosheets, allowing a better utilization of the surface active sites.

27

However, hollow structure, as an important part of

nanoparticles, has not been fabricated on graphene nanosheets widely, probably due to the sophisticated progress for constructing the hybrids. Herein, we report on the synthesis and characterization of Ni-Co LDH hollow nanocages

deposited

on

commercial

graphene

nanosheets

derived

from

ZIF-67/graphene pristine material via a structure-induced anisotropic chemical etching at elevated temperature. The overall process consists of precipitation reaction to obtain metal organic frame/graphene composite, and acidic etching route to transform template into hollow structure. The as-prepared Ni-Co LDH/graphene composite exhibits superior electrochemical performance for battery-type electrode. The conception of such sandwich like hollow structure for positive electrode has various advantages. Particularly, such hollow structure composing of multiple LDH nanosheets interconnected each other, not only provides more active sites for reacting with electrolyte but also effectively accommodates the volume expansion and



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contraction during long term charge discharge test. Moreover, graphene nanosheets, which are regarded as substrates for immobilizing hollow LDH nanocages, provide efficient paths for electron transportation. Thanks to the special sandwich like structure, the hollow nanocages grow uniformly on graphene nanosheets, and graphene nanosheets are sandwiched with hollow cages, which are feasible for fast electron transfer and reduce the ion-diffusion path. The effect of graphene mass on the electrochemical performance is also investigated and compared to the individual Ni-Co LDH nanocages, demonstrating the vital role of graphene. Therefore, we propose Ni-Co LDH nanocages/graphene composite as a viable and potential candidate for high performance asymmetric supercapacitor. 2 Experimental section 2.1 Synthesis of ZIF-67/graphene composite The commercial graphene powder with different mass (8 mg, 15 mg, 25 mg) is dispersed in 50 mL methanol under ultrasonication for 1 h. 0.498 g cobalt nitrate hexahydrate is dissolved in above solution and treated under ultrasonication for 1 h. Then 0.656 g 2-methylimidazole is dispersed in 50 mL methanol and mixed with GO/Co(NO3)2 solution. The mixture is stirred for 0.5 h and kept static at room temperature for 24 h. The product is collected by centrifugation, washed with ethanol three times, and dried at 60oC for 24 h. ZIF-67 monomer is synthesized via the same method without adding graphene powder. 2.2 Synthesis of Ni-Co LDH nanocages/graphene composite The as-fabricated ZIF-67/graphene composite 40 mg is redispersed in 50 mL ethanol containing 0.2 g Ni(NO3)2·6H2O and refluxed for 1 h under stirring. The purple powder changes to light green. The product is collected by centrifugation, rinsed with ethanol three times and dried in an oven at 60oC for 24 h. According to


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chemical treatment by weighting mass change before and after acid etching, which can remove Ni-Co LDH, the mass ratio with different graphene addition can be obtained. The mass ratios of Ni-Co LDH with 8 mg, 15 mg, 25 mg graphene are calculated to be 81%, 59% and 25%, respectively. For contrast, pure Ni-Co LDH nanocages and graphene composite fabricated by mixture is also prepared and denoted as Ni-Co LDH/graphene M. The mass ratio of Ni-Co LDH/graphene M is the same as Ni-Co LDH/15 mg graphene (59%). 2.3 Characterization The crystalline structure of the fabricated samples are determined with a powder XRD system (Rigaku TTR-III) equipped with Cu Ka radiation (λ=0.15406 nm) and Raman spectroscopy (Jobin-Yvon HR800 micro-Raman system). The morphologies of the hybrids are investigated with the aid of a scanning electron microscope (SEM, JEOL, JSM-6480) and transmission electron microscope (TEM, JEOL 2010) with a field emission gun operating at 200 kV. The energy dispersive X-ray spectrometry (EDS, JEOL JSM-6480A microscope) are used to investigate the elemental composition of the samples. The spectra of materials are determined by X-ray photoelectron spectroscopy (XPS; PHI5700 ESCA spectrometer with Al KR radiation (hυ=1486.6 eV). Nitrogen adsorption–desorption isotherms are measured to obtain surface area and porosity distribution using a micromeritics ASAP 2010 instrument. 2.4 Electrochemical characterization The electrochemical performance of Ni-Co LDH composite are performed on an electrochemical workstation CHI660I (Chenhua, Shanghai, China) via a traditional three-electrode system in which as prepared electrode hybrids (1.0 cm×1.0 cm), Pt



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foil and saturated calomel electrode (SCE) are used as the working electrode, counter electrode and reference electrode, respectively. Cyclic voltammetry (CV), galvanostatic charge discharge (GCD) and electrochemical impedance spectroscopy (EIS) measurements are performed in 1 M KOH aqueous at room temperature. 2.5 Asymmetric supercapacitors An aqueous asymmetric supercapacitor device is fabricated with Ni-Co LDH/3D RGO NF, commercial activated carbon (AC) and cellulose acetate membrane as positive electrode, negative electrode and separator respectively. The negative electrode is prepared by mixing the AC, acetylene black and polytetrafluoroethylene (PTFE) in a weight ratio of 85: 10 : 5 in ethanol under ultrasonic condition to produce a homogeneous paste. Then the paste is coated onto a piece of 1×1 cm2 nickel foam. At last, the negative electrode is pressed and dried under vacuum at 60 °C overnight. The positive electrode is prepared by the same approach with mixing Ni-Co LDH/graphene composite. The charge balance theory is used to determine the mass of negative electrode (AC), as shown in equation (1):

q  C  V  m

(1)

To obtain q+=q-, the mass balancing follows the equation (2):

m C  V  m C  V

(2) C+, C-, calculated by discharge curves, are the specific capacitances of Ni-Co LDH/graphene electrode and AC electrode respectively. △ V+ and △ V- are the potential range of Ni-Co LDH/graphene and AC electrodes, respectively. The optimum weight ratio between the Ni-Co LDH/graphene and AC electrodes is calculated to be m+/m-≈ 0.25. The active mass of positive electrode is 3.47 mg, thus the active mass of negative electrode is calculated to be 13.86 mg.


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The specific capacitance (Cs) is calculated by the following equation derived from GCD curves28.39 Cs=I ×△ t/ △V × m

(3)

The corresponding energy density E (Wh kg−1) and power density P (W kg−1) are calculated from the following equations (4, 5): E  0.5  Cs  ΔV 2

(4)

P  E  3600 / Δt

(5)

Where Cs (F g-1) is the specific capacitance; △V (V) is the potential window of the assembled device; m (g) is the active mass of electrode ; and △t (s) is the discharge time derived from charge/discharge measurement. 3 Results and discussion A schematic illustration for the synthesis of hierarchical sandwiched Ni-Co LDH hollow nanocages/graphene composite is displayed in Figure 1. The graphene nanosheets are designed to serve as deposition substrate and electron transportation scaffold. During synthesis procedure, the cobalt ions are gradually adsorbed on the surface of graphene nanosheets and subsequently react with 2-methylimidazole. The intermediate ZIF-67/graphene composite is collected and dispersed in ethanol. In the next step, the ZIF-67 polyhedrons are converted to hollow nanocages constructed from Ni-Co LDH nanosheets with the addition of Ni(NO3)2. The restriction of low conductivity for LDH can be easily overcome in our rational design by the addition of graphene nanosheets. After reaction, both hollow LDH nanocages and graphene nanosheets can remain simultaneously in composite with hierarchical structures, which can greatly boost the electrochemical performance of the material.



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Figure 1. Schematic illustration of the synthesis procedure of Ni-Co LDH hollow nanocages/graphene composite

The crystallographic structure of ZIF-67, graphene and various Ni-Co LDH/graphene composites in terms of different graphene mass, are determined by X-ray diffraction measurement (XRD). As depicted in Figure 2a, all the diffraction peaks of as-prepared ZIF-67 can be well assigned to the simulated peaks published in previous report, indicating the high purity of ZIF-67 templates.

24,29

After reacting with

Ni(NO3)2, all the diffraction peaks are well indexed to Ni-Co LDH phase (JCPDS#40-0416).

25, 30

No additional reflections are detected, revealing that the

ZIF-67 templates are completely converted to Ni-Co LDH under a mild reaction condition (Figure 2b). It is worth mention that a sharp diffraction peak located at about 26o with high intensity is observed, which is assigned to (002) plane of graphene, suggesting the high graphitization degree of graphene nanosheets. 31 Except for the high intensity peaks, a broad peak located at the same place, which is characteristic of exfoliated nature of graphene nanosheets. 32 A series of XRD patterns of Ni-Co LDH with different graphene mass are further investigated in Figure 2c. All samples exhibit diffraction peaks of Ni-Co LDH, confirming the successful transformation of ZIF-67 to Ni-Co LDH. The peaks derived from graphene


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nanosheets are revealed when the additive mass is more than 8 mg. This phenomenon is speculated due to the result that hierarchical Ni-Co LDH hollow nanocages are uniformly deposited on the surface of graphene, effectively avoid the restacking of graphene. Increasing amount of graphene nanosheets, to some extent, results in graphene agglomerated together, which is reflected in enhanced intensity of XRD peaks indexed to graphene. Note that all the diffraction peaks of the Ni-Co LDH are broaden in width and weaken in intensity, suggesting that the as-obtained Ni-Co LDH hollow nanocages are essentially composed of numerious LDH nano-crystallites. 33 The Raman spectra of commercial graphene, Ni-Co LDH and Ni-Co LDH/graphene are shown in Figure S1. Graphene and Ni-Co LDH/graphene all exhibit two obvious peaks located at 1347 cm−1, 1579 cm−1 which are the characteristic peaks of D and G bands, respectively. The G band is related to the graphitic order, and the D band is ascribed to the degree of disorder in the structure. 34 Obviously, the commercial graphene display high degree of graphitization, which due to the processing technique without involving chemical oxidation reaction. Additionally, two peaks at 470 and 520 cm−1 can be identified in Ni-Co LDH and Ni-Co LDH/graphene composites, which are due to the Ni–OH/Co–OH and Ni–O/Co–O stretching modes, respectively.

35-37

The results indicate the successfully

fabrication of Ni-Co LDH/graphene composite.



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Figure 2. XRD patterns of as fabricated (a) ZIF-67 and simulated lines, (b) graphene and Ni-Co LDH/25 mg grphene composite, and (c) Ni-Co LDH/graphene composite with different graphene mass.

For comparison, SEM images of ZIF-67/graphene composite and Ni-Co LDH/graphene composite are investigated. As depicted in Figure 3a-b, the as-fabricated ZIF-67 particles are deposited uniformly on graphene nanosheets, exhibiting a sandwich structure. The ZIF-67 particles have homogeneous polyhedron shape with smooth surface in average particle size of about 500 nm. When the ZIF-67 react with Ni(NO3)2 at an appropriate amount, the solid polyhedrons converted into hollow nanocages which can be recognized by the change of contrast (Figure 3c-d). Obviously, the as-fabricated Ni-Co LDH hollow nanocages with rough surface, retain the main structure of ZIF-67 without obvious fracture, indicating the successful fabrication of hollow structure. It is speculated that the formation process involves mild reaction conditions depicted as a diffusion-controlled ion exchange process, which is used largely to construct hollow structure from MOF materials.

38

To further

detect element distribution of Ni-Co LDH/graphene composite, energy-dispersive X-ray spectroscopy (EDS) is performed, as shown in Figure S2. The elements of selected area are C, O, Ni and Co. The C is derived from graphene nanosheets. The other elements are come from Ni-Co LDH. The element ratio of Ni and Co is calculated to be 3.2:1.8. SEM mapping results reveal that nickel and cobalt are the main elements in composite and uniformly distributed in composite (Figure 3e-g). SEM and XRD patterns of Ni-Co LDH/graphene M and Ni-Co LDH/15 mg graphene are also carried out to discuss the morphology and structure change. As shown in Fig. S3, XRD patterns of Ni-Co LDH/graphene M and Ni-Co LDH/15 mg graphene are almost the same with obvious diffraction peaks derived from LDH and graphene. As shown in Figure S4a-b, the composite prepared by mixing Ni-Co LDH and graphene show obvious distribution separately. The graphene nanosheets exhibit ultrathin layer with smooth surface, indicating that no materials distributed on graphene. However,


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Ni-Co LDH/graphene composite fabricated via in-situ growth exhibit that nanocages deposite on both surface of graphene nanosheets without obvious aggregation. During the design of material, the graphene is regared as a substrate which provides high conductivity of electrode. Moreover, the synergestic effect between Ni-Co LDH and graphene may enhance the electrochemical performance of materials.

Figure 3. SEM images of (a-b) ZIF-67/graphene, (c-d) Ni-Co LDH hollow nanocages/graphene composite and (e-g) mapping result of Ni-Co LDH hollow nanocages/graphene composite More detailed morphological information of Ni-Co LDH/graphene composite is performed by TEM. Apparently, hierarchical hollow nanocages on graphene nanosheet are detected as illustrated in Figure 4. It is easily to distinguish hollow nanocages which are attached on ultrathin graphene, with interlinked petals on the bottom and hollow nanocages on the top (Figure 4b). The phenomenon may due to the nucleation process involve in ion nucleation on the surface of graphene and nuclear growth to form polyhedron as time goes on. Subsequently, the small petals and hollow nanocages occur after ion-exchange reaction. Such hierarchical structure is feasible to expose more active surface for reacting with electrolyte, which can store numerous electric energy for applications. After meticulous observation of structure from the edge of nanocages, the shell of hollow structure is composed of



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interconnected nanosheets with ultrathin thickness (Figure 4cd). Nevertheless, the main shape of Ni-Co LDH remains intact, consistent with SEM results. The hierarchical sandwich like structure can expose more active surface for reacting with electrolyte. The surface areas of Ni-Co LDH with different graphene mass are investigated by N2 adsorption-desorption measurement. Typical IV isotherms with H3-type hysteresis loops (P/P0>0.4) can be obtained in all composites, which is the characteristic of mesopores (Figure S5). These mesopores derive from interconnected Ni-Co LDH hollow nanocages. The Brunauer-Emmett-Teller surface area of Ni-Co LDH/graphene are estimated to be 126.8, 142.8, 218.9 and 151.3 m2g−1 for various graphene mass 0 mg, 8 mg, 15 mg and 25 mg, respectively. It is obvious that the surface area of Ni-Co LDH/15 mg graphene is much higher than that of Ni-Co LDH. This is because Ni-Co LDH grows on both sides of commercial grpahene, preventing agglomeration of both Ni-Co LDH nanocages and graphene. When increase the mass of graphene higher than 15 mg, the Ni-Co LDH nanocages are insufficient to deposite on the surface of graphene, resulting in agglomeration of graphene itself, leading to significant decrease of surface areas.

39

Furthermore,

according to the pore size distribution of four samples, four samples all show mesopores centered at 3.7 nm which may derive from LDH nanosheets. When the graphene mass increase more than 15 mg, other mesopores located at about 5-20 nm occur, which may due to the interconnected graphene nanosheets. Although the characteristic results confirm the successful fabrication of LDH composite, however, to our best knowledge, LDH has general chemical formula [MII1-xMIIIx(OH)2]x+[An-]x/n·mH2O, where MII and MIII are divalent and trivalent cations, and An- can be almost any organic or inorganic anion40, 41.

30, 31

Thus, the

essential factor in constructing LDH structure is elements valences, which is of great importance in fabrication of Ni-Co LDH. In this aspect, we propose the probably reactions occurred during ion exchange process.


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Figure 4. TEM images of Ni-Co LDH hollow nanocages/graphene composite with different magnification

For detailed comparison, the valence changes before and after ion-exchange reaction is measured by XPS measurement. As illustrated in Figure 5a, the main peaks at 780.98 and 796.80 eV belongs to Co2p3/2 and Co2p1/2, with a spin-energy separation of about 16 eV, can be well assigned to Co2+ in ZIF-67. After ion-exchange reaction, the Co2p spectrum of Ni-Co LDH has been altered significantly, accompanying with two types of Co species clearly detected, which is attributed to the mixed valence of Co2+ (binding energy located at 782.92 and 798.91 eV) and Co3+ (binding energy at 780.34 and 795.14), suggesting the coexistence of Co2+ and Co3+. 32, 33, 42, 43

More importantly, the high-resolution XPS spectrum of Ni 2p is also best

fitted by considering two spin-orbit doublets characteristic of Ni2+ (binding energy at 855.64 and 873.31 eV) and Ni3+ (binding energy at 859.19 and 876.49 eV) and two shake-up satellites (Figure 5c).

44

Such observation is important, which could well



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explain the probably mechanism during synthesis procedure (Figure 5d). At initial stage, the main valence of Co in ZIF-67 is +2, however, a substantial change of valence involved in ion-exchange reaction with Ni(NO3)2 is observed, mainly represented that the main valences of Co in Ni-Co LDH are Co2+ and Co3+.45 During this procedure, it is supposed that, protons derived from the hydrolysis of Ni2+ ions can gradually etch ZIF-67 templates, leaving the hollow interior behind. The released Co2+ ions from ZIF-67 and Ni(NO3)2 may be partially oxidized by dissolved O 2 and NO3− ions in the solution quickly. At last, the Ni2+/Ni3+ and Co2+/Co3+ could coprecipitate with OH- to form Ni-Co LDH nanosheets on the surface.

46, 47

Thus,

hollow nanocages composed of Ni-Co LDH nanosheets on graphene nanosheet were obtained via such a facile method.

Figure 5. High resolution XPS spectra of (a) Co2p in ZIF-67 and (b-c) Co2p, Ni2p in Ni-Co LDH, (d) Schematic illustration of the possible mechanism reaction involved in forming Ni-Co LDH.

The electrochemical properties of ZIF-67 and Ni-Co LDH/graphene, are initial investigated by cyclic voltammetry (CV), galvonostatic charge discharge curves


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(GCD) and EIS measurement in 1M KOH electrolyte. Figure 6a presents the CV curves of ZIF-67 and Ni-Co LDH/15mg graphene composite at a constant sweep rate of 5 mV s−1. Compared with ZIF-67 with relatively low current response, Ni-Co LDH/15mg graphene material displays obvious redox peaks which can be expressed by faradic reactions below: 48 Co(OH)2 + OH ↔ CoOOH + H2O+e-

(6)

CoOOH + OH ↔ CoO2 + H2O+e-

(7)

Ni(OH)2+ OH ↔ NiOOH +H2O+e-

(8)

It is noteworthy that the integrated area of CV curves of Ni-Co LDH/graphene composite is apparently larger than that of pristine ZIF-67, suggesting Ni-Co LDH/graphene possesses ultrahigh specific capacitance. Such significantly improved electrochemical performance is further proved by galvanostatic charge-discharge curves. Figure 6b exhibits the comparison of charge-discharge curves for ZIF-67 and Ni-Co LDH/graphene composite which are measured at the same current density of 1 A g−1. Obviously, the discharge time of Ni-Co LDH/graphene (1265 F g−1) composite is much longer than that of ZIF-67 (210 F g−1), again confirming the enhanced specific capacitance of Ni-Co LDH/graphene, which is in accordance with CV results.



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Figure 6. Electrochemical performance of ZIF-67 and Ni-Co LDH/graphene composite, (a) CV curves measured at a constant scan rate of 5 mV s−1, (b) Charge-discharge curves at a current density of 1 A g −1, (c) Nyquist plots measured in the frequency range from 0.01 Hz-105 Hz.

Such superior electrochemical performance may ascribe to the low resistance of the electrode. To certify the hypothesis, EIS measurement is carried out in the frequency range from 10 kHz to 0.01 Hz. Notably, the equivalent series resistance and charge transfer resistance of Ni-Co LDH/graphene are all smaller in comparison with ZIF-67, suggesting the enhanced electrochemical properties of Ni-Co LDH/graphene. Next, we evaluate the electrochemical performance of as-prepared Ni-Co LDH/graphene composite with different graphene amount. Figure 7a displays a comparison of CV curves of Ni-Co LDH/graphene composite recorded at the same scan rate of 5 mV s−1 with 0 mg, 8 mg, 15 mg and 25 mg graphene. At a first glance, all the curves exhibit obvious redox peaks, reflecting the pseudocapacitive control of all the material. However, both materials show the similar profile with redox peaks


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located at different voltage, which may ascribed to the difference in the polarization behavior and the Ohmic resistance of the electrodes during the CV test.

49

In more

detailed, large potential ranges between redox peaks are detected in CV curves, probably due to the high resistance of material with little graphene mass (0 mg, 8 mg). Furthermore, when the graphene mass is higher than 15 mg, significant redox peaks shift associated with small redox potential range is revealed in CV curves, resulting from the synergistic effect between Ni-Co LDH and graphene nanosheets. Ni-Co LDH/15 mg graphene composite exhibits the largest CV integrated area in all the composites, suggesting the high specific capacitance at this optimal graphene mass. Continuing to increase graphene mass results in a decrease in capacitance, probably assigned to the low Ni-Co LDH loading content, which will decrease the faradic reaction sites of material to react with electrolyte. However, Ni-Co LDH with 25 mg graphene also displays obvious redox peaks with small potential range, indicating the high conductivity of this composite with excellent reversibility. As shown in Figure 7b, GCD curves are performed to further evaluate the properties of all composites at a constant current density of 1 A g−1. The potential range applied in charge-discharge curves is 0-0.6 V, which is smaller than CV curves (0-0.8V), probably due to the oxidation evolution reaction. With the increase of graphene amount, the internal resistance (IR drop) reduces dramatically, demonstrating that the graphene nanosheets can effectively reduce the resistance of composite, as well as improving the charge transport and electron collection rates. Notably, the internal resistance, obtained from GCD curves, of 25 mg graphene is much smaller than others, which may ascribe to excess amount of graphene reduce the resistance of composite. All the curves have typical plateau in accordance with CV curves, which is the characteristic of pseudocapacitance. By calculating the discharge time, the specific capacitances of Ni-Co LDH with different graphene mass are 787.6 F g−1 (0 mg), 963.2 F g−1 (8 mg), 1265.2 F g−1 (15 mg), 1020.3 F g−1 (25 mg), respectively. The high performance of Ni-Co LDH/graphene composite is tightly connected with the synergistic effect between Ni-Co LDH and graphene. For comparison, the electrochemical performance of Ni-Co LDH, Ni-Co LDH/15mg graphene and Ni-Co LDH/graphene M are



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performed, as shown in Figure S6. Ni-Co LDH/15mg graphene exhibits the highest specific capacitance with lowest resistance in three materials. Synergistic effect between graphene and Ni-Co LDH is very important to improve the performance of material. That’s why the composite synthesized by mixture of Ni-Co LDH and graphene exhibit unsatisfied performance in comparison with Ni-Co LDH/graphene fabricated by in-situ growth. To prove the existence of synergistic effect between graphene and Ni-Co LDH, detailed comparison of C1s, Ni2p and Co2p XPS spectra are illustrated in Figure S7. According to the results of XPS spectra before and after hybrid, the shifts of Co 2p, Ni 2p, and C1s peaks in binding energies confirm strong electron interactions between Ni-Co LDH and graphene nanosheets in the Ni-Co LDH/graphene composite. These interactions will change electronic states of Co, Ni and C atoms and will enhance the electrochemical performance of Ni-Co LDH/graphene. 50, 51

Figure 7. (a) CV curves of Ni-Co LDH/graphene with different graphene mass at constant scan rate of 5 mV s−1, (b) GCD curves of Ni-Co LDH/graphene with different graphene mass at current density of 1 Ag −1.


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Figure 8. (a) CV curves of Ni-Co LDH/15 mg graphene composite at different scan rates ranging from 2-20 mV s−1,(b) GCD curves of Ni-Co LDH/15 mg graphene composite at various current densities, (c) Specific capacitance calculated from discharge curves at differernt current densities, (d) Nyquist plots and (e) Cycling stability tests of Ni-Co LDH/graphene composite with different graphene mass, (f) Schematic of the electrons transportation of the as-prepared electrode The true electrochemical surface area of the Ni-Co LDH/graphene composite and pristine Ni-Co LDH electrodes is determined by a mature method which is widely utilized in oxygen evolution reaction catalyst.

52-57

All the electrodes are assumed as

pure double layer type, in whose CV curves the current density at a specific voltage should be in proportion with scan rate as illustrated in equation (9). 58 I = v × CDL

(9)

Figure S8a-d show the CV curves of Ni-Co LDH/graphene composite with different graphene mass recorded in different potential regions at various sweep rates in 1 M KOH solution. The double layer charging current is measured from the CV curves at-0.55 V, -0.52 V, -0.45 V, and -0.45 V for Ni-Co LDH, Ni-Co LDH/8 mg graphene, Ni-Co LDH/15 mg graphene and Ni-Co LDH/25 mg graphene, respectively. The anodic charging current densities are plotted against different scan rates, which give linear relation (Figure S9). The slope of liner is equal to the double layer



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capacitance. The measured double layer capacitance of Ni-Co LDH and Ni-Co LDH/graphene compsite electrodes are 1.4, 2.1, 2.7 and 2.7 mF cm2, respectively. Ni-Co LDH with 15 mg and 25 mg graphene exhibit high ECSA in comparison with other two electrodes. The results indicate that graphene can effectively increase ECSA of electrodes. Detailed CV profiles of Ni-Co LDH/15 mg graphene composite, measured at various scan rates ranging from 2 to 20 mV s−1 in potential range between -0.2 to 0.8 V, are plotted in Figure 8a. As observed from CV curves, no significant change is detected in the shapes of CV curves, thus suggesting good rate capability of the Ni-Co LDH/15 mg graphene electrode. The scan rates gradually increase accompanying with redox peaks shift to more positive and negative potential, which is due to the internal resistance of the electrode59.41 Figure 8b displays the GCD curves of Ni-Co LDH/15 at various current densities from 1 A g−1 to 10 A g−1. Observation of the obvious voltage plateaus during charge/discharge profiles further confirms the faradic behavior of material, in consistent with the CV results. The specific capacitance as a function of current densities of Ni-Co LDH with different graphene mass is plotted in Figure 8c. For Ni-Co LDH/15 mg graphene composite, the maximum specific capacitance can reach 1265 F g−1 and still retain 637.3 F g−1 (about 50.1% capacitance retention) after current density increase from 1 A g−1 to 10 A g−1. Additionally, such capacitance retention can increase to 52.3% for 25 mg graphene composite when current density increase ten times and at this graphene mass, the maximum specific capacitance is 1020 F g−1. However, for the samples with low graphene amount and without graphene, the capacitance are so limited such as 59 % and 48 % capacitance retention for Ni-Co LDH with 8 mg and 0 mg graphene when current density increase only 3.33 times, respectively. The results demonstrate that the graphene amount plays an important role in promoting the electrochemical performance of Ni-Co LDH,


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including conductivity and rate capability. The rate capability can be improved with the increase of graphene mass, presumably due to the high conductivity of graphene nanosheets. The addition of graphene can reduce electron diffusion path, thus decrease the overall resistance of the composite. The enhanced electrochemical performance depending on the graphene mass is also confirmed by EIS measurements as shown in Figure 8d. All the plots are composed of semicircle in the high frequency, followed by a straight line in low frequency region. The intercepts on the real axis representing equivalent series resistance (ESR) of four samples are 10.24 Ω (0 mg graphene), 7.6 Ω (8 mg graphene), 6.1 Ω (15 mg graphene) and 4.9 Ω (25 mg graphene), respectively. The result indicates that the more graphene mass, the lower internal resistance can be obtained. Moreover, the diameter of semi-circles gradually decreases with the increase of adding graphene mass, which implies feasible electron transportation. The more vertical straight plots at low-frequency region with high graphene mass, indicates that Ni-Co LDH/graphene possesses ideal capacitive and fast ion diffusion behaviors. As a result, the electrochemical resistance of Ni-Co LDH can be greatly reduced by the addition of graphene, leading to the enhanced electrochemical performance of materials. To validate the importance of graphene mass regarding the cycling stability, continuous charge-discharge 2000 circle are performed to Ni-Co LDH with various graphene mass at a constant current density of 3.33 A g−1 (Figure 8e). Obviously, the activation of electrode and electrolyte gradually penetration into the hollow nanocages during cycling occur as evidenced by gradual increase in specific capacitance during the initial 200 circles. After 200 circles, the capacitance presents gradually decay in varying degrees, supposed to be caused by the instability of Ni-Co LDH and fracture of hollow structure during long-term charge-discharge test. With respect to the four



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samples, Ni-Co LDH without graphene only achieve 42.1% capacitance retention after cycling 2000 circle, which is much smaller than those of 8 mg (64.5%), 25 mg (91.8%). Especially, the capacitance can still be retained 92.9 % when the graphene adding mass is 15 mg. As can be seen, the cycling stability of Ni-Co LDH is largely dependent on mass of graphene, which is considering that the Ni-Co LDH manifests low conductivity that is not feasible to ion-diffusion. However, more graphene nanosheet can restack together when the adding mass is ultrahigh. The restacking graphene can hardly decrease the contact surface for reacting with electrolyte, thus reduce the specific capacitance. In this case, 15 mg is an optimal amount in design of Ni-Co LDH/graphene composite for high performance battery-type electrode with high specific capacitance, rate capability and excellent cycling stability. The reason for such high electrochemical performance of the composite can be concluded as follows: First, Ni-Co LDH with hollow nanocages structure can provide more active sites for faradic reactions. Moreover, such hollow structure can also accommodate the volume expansion and contraction arising from long-term charge-discharge test. 42, 62 Second, graphene nanosheets used as a substrate for immobilization of Ni-Co LDH hollow structure can effectively restrict the aggregation of Ni-Co LDH nanocages and restacking of graphene nanosheets. Such design can expose more active sites for reacting with electrolyte. Third, graphene, with high conductivity, used as scaffold, can transport the electrons in high rate, thus decrease the overall resistance of electrode.


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Figure 9. (a) Schematic of the assembled asymmetric supercapacitor with Ni-Co LDH/15mg graphene composite as positive electrode and active carbon as negative electrode, (b) CV curves of Ni-Co LDH/15 mg graphene and active carbon at a constant sweep rate of 2 mV s−1, (c) CV curves of assembled supercapacitor devices measured at different scan rates ranging from 2 mV s −1 to 40 mV s−1, (d) Galvanostatic charge-discharge curves of fabricated asymmetric supercapacitor at various current densities, (e) Cycling performance of assembled supercapacitor during 2500 circles at a constant current density of 1.4 A g −1, inset: charge-discharge curves of assembled device, (f) Ragone plot of assembled supercapacitor device, inset: the photo of assembled supercapacitor and the lighted up LED.

The suitability of as-prepared material in practical utilization is also evaluated in two-electrode asymmetric supercapacitor consisting of Ni-Co LDH/15 mg graphene as positive electrode and active carbon as negative electrode. As illustrated in Figure 9a, PTFE membrane is sandwiched with two electrodes and 1M KOH is used as electrolyte. Before testing the electrochemical performance of the device, the operating voltage range is surmised by measuring the CV curves of Ni-Co LDH/15mg graphene and active carbon respectively. As shown in Figure 9b, the CV curve of



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active carbon, measured between potential range between -1 and 0 V, presents quasi-rectangle without redox peaks resulted from the characteristic of double layer capacitive behavior. In the contrary, obvious redox peaks of Ni-Co LDH/15mg graphene composite are observed with a potential range -0.2-0.8 V derived from faradic reactions. Due to the obvious polarization phenomenon occurred when the potential exceed 0.7 V, the optimal potential range of asymmetric supercapacitor is determined to be 0-1.7 V. Such potential range is higher than previous reports, which is competitive to increase energy and power densities. Figure 9c displays the CV curves of the assembled device measured at various sweep rates ranging from 5 mV s−1 to 40 mV s−1. A combination of both pseudocapacitance in high voltage and EDLC types in low voltage of capacitance have been observed at all sweep rates. Furthermore, the redox peaks shift to more positive and negative potential with the increase of scan rates. However, the shape of CV curves don’t exhibit obvious distortion although the scan rate is as high as 40 mV s−1, indicating the high rate capability and rapid I-V response. Galvanostatic charge-discharge curves are also performed to evaluate the specific capacitance of asymmetric device. As shown in Figure 9d, the shapes of charge-discharge curves tend to be quasi-rectangle, and charge curves are nearly symmetric with the discharge parts, suggesting excellent capacitive behavior of the assembled device. Based on the discharge time of charge-discharge curves, the maximum specific capacitance of the asymmetric device can reach 170.9 Fg−1 at current density of 0.7 A g−1, although the current density increases eight times, the specific can still retain 60.2 F g−1. Moreover, the assembled supercapacitor device also exhibits exceptional cycling stability which is measured by repeating charge-discharge curves. After 2500 circle, the electrode can still maintain 94.2% of initial capacitance value, again confirming


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the potential of such material in supercapacitor. The asymmetric supercapacitor still exist some drawbacks (high IR drop from charge-discharge curves) derived from capacitance mismatch of positive and negative electrodes. However, the assembled device can deliver maximum energy density of 68.0 Wh kg−1 at a power density of 594.9 W kg−1 and still retains 23.96 Wh kg-−1 at a power density of 4759.1 W kg−1, as illustrated in the Ragone plot which reveals the relationship between energy and power densities. The detailed electrochemical performance are also listed in Table 1. Such energy and power output are much higher than previous reports about asymmetric supercapacitors, such as Ni-Co DH//AC,61 rGO/CoAl-LDH//RGO,50 MnCo-LDH@Ni(OH)2//AC,62 NiCo2O4@NiO//AC, 64

Ni-Co

LDH

from

ZIF-67//AC,

[email protected]//CBC-N2.

67

65

63

CoNiFe-LDH/CNFs-0.5//AC,

HCNs@NiCo-LDH//graphene,

66

Moreover, the asymmetric supercapacitor device

with size of 1×1 cm2, can light up a LED as illustrated in inset of Figure 9f, again demonstrating the potential for utilization in sustainable energy-storage conversion devices. Table 1 Comparison of electrochemical performances for various supercapacitors Positive materials//Negative materials

Energy density (Wh kg-1)

Power density (W kg-1)

Cycle number

Retention%

Ni-Co DH//AC

42.5

400.0

3000

80.7

61

rGO/CoAl-LDH//RGO

22.6

90.0

5000

94.0

50

MnCo-LDH@Ni(OH)2//AC

47.9

750.7

5000

90.9

62

NiCo2O4@NiO//AC

31.5

215.2

3000

89.0

63

CoNiFe-LDH/CNFs-0.5//AC

30.2

800.1

2000

82.7

65

Ni-Co LDH from ZIF-67//AC

27.5

375.0

1000

89.3

66

HCNs@NiCo-LDH//graphene

47.04

699.7

3000

93.5

67

[email protected]//CBC-N2

36.3

800.2

2500

89.3

68

Ni-Co LDH/graphene//AC

68.0

594.9

2500

94.2

This

Ref

work

Conclusion



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In summary, hierarchical Ni-Co LDH hollow nanocages incorporated with graphene nanosheets via facile in-situ precipitate followed by ion-exchange reaction is reported, which is easy to control and suitable to large-scale production. The as-synthesized composite shows hollow structure composed from interconnected LDH nanopetals, homogeneously deposited on graphene nanosheets. The special hierarchical hollow structure associated with high conductive graphene facilitates the transmission of electrolytic ions and guarantees a more efficient charge and higher redox capacitance. Moreover, the composite with optimal graphene mass of 15 mg reveals excellent electrochemical results as battery-type electrode. The assembled asymmetric supercapacitor based on composite and active carbon exhibits excellent electrochemical performance, including high specific capacitance, energy density and power density and good cycling stability. The electrochemical results indicate that Ni-Co LDH/graphene is a promising material for battery-type electrode in sustainable energy devices. Supporting information Raman spectra of graphene, Ni-Co LDH and Ni-Co LDH/graphene; EDS spectrum of Ni-Co LDH/graphene composite; XRD patterns and SEM images of Ni-Co LDH/graphene M and Ni-Co LDH/15 mg graphene; Nitrogen adsorption-desorption isotherms and BJH pore size distributions of Ni-Co LDH with different graphene mass; Electrochemical performance of Ni-Co LDH, Ni-Co LDH/graphene Mix and Ni-Co LDH/15 mg graphene; XPS spectra of graphene and Ni-Co graphene composite; CV curves of Ni-Co LDH with different grapheen mass; Current density as a function of the scan rate for all prepared electrodes. Acknowledgements This work was supported by National Natural Science Foundation of China (NSFC


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61473095), Fundamental Research Funds of the Central University (HEUCFM), Natural Science Foundation of Heilongjiang Province（B2015021）, International Science & Technology Cooperation Program of China (2015DFR50050) and the Major Project of Science and Technology of Heilongjiang Province (GA14A101).

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Table of Contents (TOC image) Sandwiched Ni-Co LDH hollow nanocages/graphene composite was fabricated via facile in-situ precipitate followed by ion-exchange reaction. The special hierarchical hollow structure associated with high conductive graphene facilitated the transmission of electrolytic ions and guaranteed a more efficient charge and higher redox capacitance. Moreover, the assembled asymmetric supercapacitor based on composite and active carbon exhibited excellent electrochemical performance, indicating promising potential

of Ni-Co LDH/graphene

composite

electrochemical energy devices.


for

electrode

in


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

Figure 1


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Figure 2



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

Figure 3


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Figure 4



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

Figure 5


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Figure 6



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

Figure 7


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Figure 8



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

Figure 9


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Positive materials//Negative materials

Energy density (Wh kg-1)

Power density (W kg-1)

Cycle number

Retention%

Ni-Co DH//AC

42.5

400.0

3000

80.7

61

rGO/CoAl-LDH//RGO

22.6

90.0

5000

94.0

50

MnCo-LDH@Ni(OH)2//AC

47.9

750.7

5000

90.9

62

NiCo2O4@NiO//AC

31.5

215.2

3000

89.0

63

CoNiFe-LDH/CNFs-0.5//AC

30.2

800.1

2000

82.7

65

Ni-Co LDH from ZIF-67//AC

27.5

375.0

1000

89.3

66

HCNs@NiCo-LDH//graphene

47.04

699.7

3000

93.5

67

[email protected]//CBC-N2

36.3

800.2

2500

89.3

68

Ni-Co LDH/graphene//AC

68.0

594.9

2500

94.2

This work

Table 1


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