ZIF-derived wrinkled Co3O4 polyhedra supported on 3D macroporous carbon sponge for supercapacitor electrode

Accepted Manuscript ZIF-derived wrinkled Co3O4 polyhedra supported on 3D macroporous carbon sponge for supercapacitor electrode Xiao Han, Shitai Liu, Luolin Shi, Shuang Li, Ye Li, Yukun Wang, Wen Chen, Xiaoran Zhao, Yan Zhao PII:

S0272-8842(19)31007-7

DOI:

https://doi.org/10.1016/j.ceramint.2019.04.182

Reference:

CERI 21376

To appear in:

Ceramics International

Received Date: 21 February 2019 Revised Date:

20 April 2019

Accepted Date: 21 April 2019

Please cite this article as: X. Han, S. Liu, L. Shi, S. Li, Y. Li, Y. Wang, W. Chen, X. Zhao, Y. Zhao, ZIFderived wrinkled Co3O4 polyhedra supported on 3D macroporous carbon sponge for supercapacitor electrode, Ceramics International (2019), doi: https://doi.org/10.1016/j.ceramint.2019.04.182. 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.

ACCEPTED MANUSCRIPT ZIF-Derived Wrinkled Co3O4 Polyhedra Supported on 3D Macroporous Carbon Sponge for Supercapacitor Electrode

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Xiao Hanǁ, Shitai Liuǁ, Luolin Shiǁ, Shuang Li, Ye Li, Yukun Wang, Wen Chen, Xiaoran Zhao, Yan Zhao*

School of Materials Science and Engineering, Beihang University, Beijing 100191,

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China

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These authors contributed equally.

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*Correspondence: [email protected]; Tel.: +86-010-8231-7127 (Y.Z.)

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Abstract: Co3O4/melamine-derived carbon sponge (MCS) nanocomposite in which wrinkled ball-in-dodecahedral Co3O4 nanoparticles derived from ZIF-67 were homogeneously dispersed on the interconnected MSC was fabricated via a simple

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immersion and thermolysis route. As-prepared ultralight Co3O4/MCS possessed

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mechanically robust characteristic and unique 3D macroporous framework anchored with corrugated Co3O4 dodecahedra. Utilized as a pseudocapacitor electrode, Co3O4/MCS hybrid exhibited a great specific capacitance of 1409.5 F g-1 at the current density of 0.5 A g-1 and excellent long-term cycling stability of 93.2% after 1000 charge/discharge cycles, which might be ascribed to the synergistic effect of the inherent high redox activity from Co3O4 polyhedra combined with excellent electrical conductivity of MCS. This work demonstrates that tunable structure design and rational morphology control are efficient

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for

manufacturing

novel

electrode

materials

with

extraordinary

electrochemical performance.

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Keywords: nanocomposite; supercapacitor; melamine; carbon sponge; Co3O4

1. Introduction

With the increasing anxiety on severe resource shortage and soaring energy

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requirement, the exploitation of sustainable and renewable energy resources has been demanded urgently [ 1 -3 ]. As a consequence, various rechargeable electrochemical

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energy storage devices have been intensively investigated to achieve the expected performance including high power/energy density, fast charge-discharge rate, long cycle life, accompanied with light weight, low cost and environmental friendliness [4,5]. Owing to their outstanding redox features and theoretical capacitive characteristics,

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transition metal oxides (TMOs) have been considered as one of the most promising electrochemical energy storage materials. In recent years, TMOs such as Co3O4, MnO2, RuO2, Fe3O4, NiMoO4 and NiCo2O4 with different compositions and structures have been

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synthesized via various approaches [6-8]. Unfortunately, when TMO itself is utilized as

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electrode material, the heavy volume variation caused by particle reunion during the charge-discharge process gives rise to poor rate capabilities as well as obvious capacity degradation [9,10]. Additionally, due to their semi-conductive features, the sluggish electron transportation impairs the electrochemical activity of TMOs, resulting in the unsatisfactory electrochemical charge storage behaviors towards practical applications. A promising solution to address the above-mentioned problems is to directly hybrid TMOs with conductive additives, especially carbon nanomaterials like graphene and carbon nanotubes. Recently researchers have made great efforts to prepare TMOs/carbon

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to

enhance

their

capacitive

performance.

However,

these

nanocomposites are normally in the powdered form, which have to be mixed with acetylene

black

and

polyvinylidene

fluoride

(PVDF)

in

the

presence

of

1-Methyl-2-pyrrolidinone (NMP) to fabricate the working electrode. The existence of

complexity and

degraded

electrochemical

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binder and strongly polar organic solvent would lead to the increased processing performance.

Thus,

a

binder-free

self-supported electrode with mechanically robust characteristic would be more

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favorable.

Apart from the hybridization with conductive carbon substrate, the morphologies of

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TMOs have also been proven to play a critical role in tuning the kinetics and thermodynamics at the interfaces between electrochemically active sites and electrolyte. Specially, so far, TMOs in nanoparticles, nanowires, nanosheets and even flower-like nanostructures have already been prepared, which exhibit different electrode-electrolyte

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interface properties and hence show dissimilar electrochemical performances [11-13]. But the precise control on the morphology of TMOs with hierarchical nanoporous structure remains a challenge.

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Herein, we propose a novel Co3O4/melamine-derived carbon sponge (MCS) with

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unique nano-/macro-porous architecture to be utilized as the electrochemical energy storage electrode. MCS is produced directly through the carbonization process of commercially available melamine foams, and Co3O4 with concave ball-in-dodecahedral structure is obtained via using zeolitic imidazolate framework-67 (ZIF-67) as the precursor. The design of this hybrid structure comes from the following considerations. As ZIF-67 is an appealing material with large specific surface areas, low density and versatile functionalities, ZIF-derived nanoporous Co3O4 crystals would possess excellent electrochemical performance by providing electrolyte ions with storage space and

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ACCEPTED MANUSCRIPT improving the accessibility of the electrolyte ions to the electrochemically active materials. Additionally, MCS works as a conductive network to boost electrical conductivity, a volume buffer to reduce the internal stress during the transformation of Co3O4 from ZIF-67 under thermal environment, and a heat conductor that effectively

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dissipates the reaction heat and lower the aggregation of active nanomaterials. The synergistic effects derived from the metallic property of TMOs and the carbonaceous property of MCS will endow the obtained Co3O4/MCS with extraordinary

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electrochemical performance.

As-synthesized Co3O4/MCS possessed an outstanding specific capacity of 1409.5 F

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g-1 at the current density of 0.5 A g-1 and an excellent cycling stability of 93.2% after 1000 charge/discharge cycles in the 1 M KOH aqueous electrolyte solution. This work might shed some new light on the design and fabrication of high-performance electrochemical energy storage devices with high power density, fast charge-discharge rate, long cycle

2.1 Materials

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2. Experimental

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life, light weight and environmental friendliness.

Cobalt nitrate hexahydrate (Co(NO3)2·6H2O) was purchased from Guangdong

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Guanghua Technology Co., Ltd, China. 2-methyl-imidazole (2-MIM, 99%) was bought from Beijing J&K Scientific Ltd. Co., Ltd, China. Methanol and ethanol were provided by Beijing Chemical Works. SDBS was offered by Shanghai Macklin Biochemical Technology Co., Ltd. Melamine sponges were provided by Greencare International (Guangzhou) Co., Ltd. All the chemical reagents were utilized as acquired without any further purification or treatment.

2.2 Synthesis of Co3O4/MCS nanocomposite 4

ACCEPTED MANUSCRIPT A piece of melamine foam with dimension of approximately 3 cm × 4 cm × 5 cm was washed with ethanol in a bath-sonicator to remove surface impurities and then dried in an oven at 70 °C overnight, which was followed by the carbonized process at 800 °C for 2 h with the heating ramp of 2 °C min-1 in argon atmosphere to obtain elastic MCS. The

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as-synthesized MCS was cut into size of 1 cm×1 cm×2 cm and then one piece was compressed and immersed into a solution obtained by dissolving 80 mg SDBS in 20 mL deionized water, followed by centrifugation at 300 rpm for 30 min to facilitate the

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deposition of surfactant on MCS. Afterwards the sponge was taken out with a tweezer

surfactant-modified MCS.

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followed by being washed repeatedly with ethanol and dried at 60 °C to acquire the

ZIF-67 was synthesized via a quick and vigorous mixture of two solutions which were firstly prepared by dissolving 1 mmol of Co(NO3)2·6H2O and 4 mmol of 2-MIM in 25 mL methanol, respectively. Then, the obtained solution was aged for 24 h at room

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temperature without any turbulence. The resultant purple sediment was collected by centrifugation with ethanol and dried at 70 °C overnight. After cooling down to room temperature naturally, 80 mg ZIF-67 was dissolved in 20 mL deionized water and the

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mixture was sonicated for 60 min to prepare ZIF-67 suspension.

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Subsequently, SDBS-modified MCS was immersed into the ZIF-67 suspension, followed by mild stir for 3 hours to allow the ZIF-67 to be self-assembled on the surface of MCS. The sponge was withdrawn carefully, washed thoroughly with ethanol and dried at 90 °C under vacuum overnight to produce ZIF/MCS. Finally, the resultant ZIF/MCS was thermolized at 350 °C for 2 hours with the heating ramp of 1 °C min-1 in the air to form Co3O4/MCS.

2.3 Characterizations

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ACCEPTED MANUSCRIPT Crystalline structures were characterized by X-ray diffraction (XRD, Rigaku D/max-rB with Cu Kα radiation, λ = 1.5406 Å). Morphologies were observed via scanning electron microscopy (SEM, JSM-7500) equipped with energy dispersive spectrometer (EDS) and transmission electron microscopy (TEM, JEM-2010F). Surface

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chemistry was measured by X-ray photoelectron spectroscopy (XPS, ESCALab 220i-XL with Al K radiation, 1486.6 eV). Raman spectra were obtained on HORIBA Jobin Yvon S.A.S. (λex = 632.8 nm). The relative content of each component was obtained by

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thermogravimetric analysis (TGA, NETZSCH STA409 C/3/F, in air).

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2.4 Electrochemical measurements

For MCS-based samples, a piece of sponge with the area of 1 cm × 1 cm was sandwiched between two pieces of Ni foams and pressed to prepare working electrode. For powdered samples, the homogeneous slurry constituting Co3O4, carbon black and

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polyvinylidene difluoride with the mass ratio of 75:20:5 in N-methyl-2-pyrrolidone was pasted onto Ni foam and dried at 110 °C under vacuum overnight. The loading mass of the active material was 1.0-1.5 mg cm-2.

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All electrochemical measurements were performed with the CHI 660E electrochemical workstation in a standard three-electrode system including a Pt foil

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serving as the counter electrode, a saturated calomel electrode (SCE) as the reference electrode and 1 M KOH aqueous solution as the electrolyte. Herein, cyclic voltammetry (CV) and galvanostatic charge-discharge (GCD) curves were tested. The specific capacitance (C (F g-1)) of the electrode could be calculated from CV curves (1) and GCD curves (2) according to the following equations:

C=

∫ IdV

(1)

vmV

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ACCEPTED MANUSCRIPT where I (A), V (V), v (mV s-1) and m (g) refer to the current, potential range of both charge and discharge process, scan rate and mass loading of the active material, respectively.

∫ Idt

(2)

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C=

mV

where I (A), t (s), m (g) and V (V) refer to the discharging current, discharging

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time, mass loading of the active material and discharging potential range,

3. Results and discussion

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respectively.

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3.1 Structural and morphological characterizations

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Figure 1. Macro characterizations of Co3O4/MCS. (a) Schematic of the fabrication process; (b)Volume contrast between melamine foam and Co3O4/MCS; (c) Light weight characteristic; (d) High elasticity characteristic; (e) Compressive curves; (f, g) Flame retardent characteristic.

The schematic of fabrication process and macro characterization results are presented in Figure 1. As shown in Figure 1(a), melamine foam is working as the starting

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ACCEPTED MANUSCRIPT materials for the synthesis of three-dimensional macroporous framework through thermal treatment. Afterwards, MCS was decorated with Co3O4 polyhedra with the assistance of surfactant and ZIF-67. The digital photos of as-prepared Co3O4/MCS are exhibited in Figure 1(b, c). After carbonization, there was a discernible shrinkage of Co3O4/MCS’s

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volume to approximately a quarter of the initial melamine foam bulk. Besides, it is worth noting that Co3O4/MCS was ultralight whose weight could be withstood by a bristlegrass without any visible deformation. Moreover, Co3O4/MCS inherits the desirable elasticity

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from melamine sponge. As demonstrated in Figure 1(d), Co3O4/MCS could be compressed with the strain of more than 80% and completely return to its original state

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without any obvious mechanical failure. The compressive curves were also tested and presented in Figure 1(e). Surprisingly, Co3O4/MCS could also be exposed to ethanol flame without any burning, possessing excellent ethanol flame retardant performance, as shown in Figure 1(f, g). Overall, as-prepared Co3O4/MCS exhibited robust mechanical

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property and unique 3D microporous structure, providing a promising platform for

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further functionalization and wider applications.

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Figure 2. Structure of Co3O4 and Co3O4/MCS. (a) XRD patterns; (b) Raman spectra; (c) Crystalline structure of ZIF-67; (d) Schematic morphology of ZIF-67; (e)

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Schematic morphology of ZIF-derived Co3O4; (f) Crystalline structure of Co3O4.

The structures of Co3O4 particles and Co3O4/MCS nanocomposites are characterized

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in Figure 2. As observed from XRD patterns (Figure 2(a)), both Co3O4 and Co3O4/MCS samples possess characteristic diffraction peaks at 31.3°, 36.8°, 38.5°, 44.8°, 55.7°, 59.4°,

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65.2°, 68.6°, 74.1° and 77.3°, corresponding to the (220), (311), (222), (400), (422), (511), (440), (531), (620) and (533) crystalline planes of spinel structured Co3O4 (JCPDS card no. 43-1003) [14], which confirms that ZIF-67 has been completely converted into Co3O4 after an annealing treatment. Besides, Co3O4/MCS consists of an additional broad diffraction peak at around 20°, which could be assigned to the (002) crystal plane resulting from the amorphous carbonaceous structure [15]. In consistence with XRD results, Raman spectra (Figure 2(b)) reconfirm that all the ZIF-67 precursors have been transformed into Co3O4. Four characteristic Raman 10

ACCEPTED MANUSCRIPT vibration peaks are exhibited at 482.4, 521.6, 618.4 and 691.0 cm-1, corresponding to Eg, F1g, F2g and A1g vibration modes originated from Co3O4, respectively [16]. Moreover, a couple of broad bands at 1330 cm-1 (D) and at 1590 cm-1 (G) belong to the typical carbon materials. Therefore, XRD and Raman analyses prove that the expected Co3O4/MCS

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nanocomposites have been synthesized successfully. Figure 2(c-f) depict the schematics

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for crystalline structure and morphology of ZIF-67 and ZIF-derived Co3O4 respectively.

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Figure 3. Compositions of Co3O4/MCS. (a) XPS Survey spectrum; (b) High-resolution XPS spectra at C 1s region; (c) N 1s region; (d) O 1s region; (e) Co 2p region; (f) TGA

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curves.

XPS was performed to elucidate the specific chemical and electronic state of the

elements in Co3O4/MCS nanocomposite. The signals from Co, C, O and N species are evident in XPS survey spectrum (Figure 3(a)), which reveals that no other impurities exist in the nanocomposite. Among the signals, C 1s peak attributed to MCS is prominent. The high-resolution XPS spectrum at C 1s region (Figure 3(b)) has been broadened by multiplet splitting effect and can be deconvoluted into four spin-orbit peaks, namely 11

ACCEPTED MANUSCRIPT sp2-hybridized graphitic carbon at 284.4 eV, sp3-hybridized diamond carbon and sp2-hybridized nitrogen-bonded carbon at 285.0 eV, surface oxygen and nitrogen groups bonded carbon at 286.2 eV and at 288.6 eV [17-19]. Meanwhile, it can be observed from Figure 3(c) that N 1s spectrum could be fitted with three spin-orbit doublets,

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corresponding to pyridinic, pyrrolic and graphitic N respectively [20]. In Figure 3(d), the O 1s XPS spectrum can be decomposed into three independent bonding features, of which the binding energy is located at 529.9, 531.5 and 533.1 eV corresponding to the

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metal-oxygen bond, defects and chemisorbed oxygen or undercoordinated lattice oxygen as well as the few amounts of physisorbed and chemisorbed water, respectively [21]. The

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high-resolution spectrum of Co 2p (Figure 3(e)) exhibits two peaks at 780.9 eV and 796.5 eV along with two shakeup satellites, corresponding to Co 2p3/2 and Co 2p1/2. The two-spin-orbit doublets of Co 2p curve are attributed to the peaks of Co2+ and Co3+, which supports the existence of Co3O4 [22]. Consequently, all the XPS results sustain the

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argument of obtaining Co3O4/MCS nanocomposite.

The thermal behaviors of MCS and Co3O4/MCS were also investigated through TGA characterization (Figure 3(f)). TGA curve of MCS indicates that MCS undergoes a

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thorough weight loss at the temperature of ~400 °C and hence it is allowed to be designed

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as the matrix material withstanding the annealing temperature of 350 °C. For Co3O4/MCS nanocomposite, ascribed to the presence of inorganic metal oxide components, the residual weight percentage remains steadily at 52.4% of the initial value.

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Figure 4. SEM images of (a, b) MCS, (c) ZIF-67, (d) ZIF-derived Co3O4 and (e, f) Co3O4/MCS.

Morphology of the as-prepared samples was observed by SEM, as presented in Figure 4. Figure 4(a) is the low-resolution SEM image of 3D interconnected MCS, a

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porous network possessing macropores with the hole size of 60-200 µm and the edge size of several micrometers. This unique macroporous structure is the main reason for the

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ultralight weight. The higher resolution SEM image in Figure 4(b) shows that surface of MCS is extremely smooth. In Figure 4(c), the uniform ZIF-67 particles show smooth

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surface and dodecahedral shape with an average edge length of approximately 500 nm. After annealing, ZIF-67 particles tend to agglomerate and every edge got blunt. And the original regular dodecahedral shape collapsed. However, with the assistance of 3D MCS framework, the pristine morphology from ZIF-67 was well preserved after annealing, as shown in Figure 4(e, f). The surface of MCS became crumpled and Co3O4 particles have been successfully anchored on it, especially on the crossover point of the skeleton. The roughness of MCS promotes the location of more nanoparticles and the framework provides a conductive path for Co3O4 to improve its capacitance activity. Moreover, 13

ACCEPTED MANUSCRIPT compared with Figure 4(d), Co3O4/MCS hybrid structure solves the problem of the nanoparticles conglomeration, which can effectively increase the specific surface area of the active substances and hence improve the electrochemical performance tremendously. This is likely because during the oxidation reaction of ZIF-67, the internal heat produced

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is stored in the stacked powder, which leads to overheating and particles aggregation. However, with the sustaining of MCS, ZIF-67 can dissipate heat timely by transmitting the heat through the carbonaceous framework with good thermal conductivity, which can

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effectively avoid reunion, the side effect of annealing. Moreover, MCS can act as volume buffer to reduce internal stress, which can also prevent agglomeration. From the

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high-resolution SEM image of Co3O4/MCS in Figure 4(f), it can be clearly observed that Co3O4 still retained the same polyhedral shape as ZIF-67 but with smaller average size of around 300 nm as well as indented and corrugated facets. The crystalline structure and morphology evolutions during the annealing process have been schematically presented

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in Figure 2(c-f). From above mentioned, not only does such hybrid configuration facilitate both electrical conduction and mass transfer, but also it provides a huge quantity of active sites for redox reactions, which enables Co3O4/MCS to be an ideal electrode

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material for supercapacitors [23].

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Figure 5. TEM images (a-c) and EDS image (d) of Co3O4/MCS.

TEM characterization of (Figure 5) further confirms that the polyhedral grains

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possess the edge length of roughly 300 nm and shriveled facets, consistent with SEM observations. The subtler construction in which ZIF-derived Co3O4 is in concave

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ball-in-dodecahedral structure is revealed. The striking contrast in the black center core illustrates the pores (indicated by the arrows) and the grey surrounding area is sandwiched between the internal core and the external dodecahedral edges. According to the previous literature [24,25], such structure attributed to the heterogeneous contraction caused by non-equilibrium heat treatment is particularly beneficial for electrochemical process by providing more energy-storage sites and cutting down the diffusion path of ions to the active sites. The porosity endows the electrochemically active material and electrolyte ions with high accessibility, while the buffer space between the core and 15

ACCEPTED MANUSCRIPT external shell is functioned as the temporary storage trough for the electrolyte ions during continuous charge/discharge process to boost long-term stability. Figure 5(b) is a detailed observation on a part of Co3O4, showing that these particles are actually composed of numerous primary crystallites (rounded by the circles) with the size of approximately 10

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nm [26], which is in good accordance with the result calculated by the Scherrer equation from the XRD pattern. The high-resolution TEM (HRTEM) image (Figure 5(c)) exhibits that the d-spacings of 0.20 nm, 0.23 nm and 0.24 nm correspond to the (440), (222) and

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(331) planes of Co3O4, respectively, which is consistent with the values estimated from XRD characterization. EDS characterization of Co3O4/MCS was conducted to analyze

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the elementary distribution and the elemental mapping images of Co, C, N are shown in Figure 5(d). The elements of Co, C, N are uniformly distributed in different regions of the nanocomposite.

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3.2 Electrochemical measurements

Figure 6. Electrochemical performance of MCS, Co3O4 and Co3O4/MCS. (a) CV curves at the scan rate of 50 mV s-1; (b) GCD curves at the current density of 1 A g-1; (c) CV curves of Co3O4/MCS at different scan rates; (d) GCD curves of Co3O4/MCS at various 16

ACCEPTED MANUSCRIPT current densities; (e) Dependence of specific capacitance on different current densities; (f) Cycling performance.

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The electrochemical performance of the synthesized substances was characterized in a three-electrode system and the results are depicted in Figure 6. Typical CV profiles for MCS, Co3O4 and Co3O4/MCS nanocomposite are compared in Figure 6(a), in which good pseudocapacitance can be observed in the two distinct and symmetrical redox peaks

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arising from the reaction between the Co2+/Co3+ redox couple, which can be described as

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the following reactions [27]:

Co3O4 + OH− + H2 O ↔ 3CoOOH + e−

(3)

CoOOH + OH− ↔ CoO2 + H2O + e−

(4)

The GCD patterns of MCS, Co3O4 and Co3O4/MCS at the current density of 1 A g-1

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are shown in Figure 6(b). The Co3O4/MCS hybrid electrode exhibits the longest charge-discharge time compared with both MCS and Co3O4 electrodes, indicating larger electrochemical charge storage capacity. The highest specific capacitance for MCS,

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Co3O4 and Co3O4/MCS is calculated from GCD curves by using the Eq. (2) as 66.5 F g-1, 593.9 F g-1 and 1317.2 F g-1 at the current density of 1 A g-1, respectively [28,29]. The

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excellent electrochemical property of Co3O4/MCS is not only derived from the 3D interconnected framework endowed conductivity and arrestment of agglomeration [30], but also benefited from the porous core-shell nanostructure induced provision of numerous active sites for the fast redox reactions and shortening of the diffusion distances between OH- and the electroactive sites because of the presence of voids, a unique structure between the core and the shell [31,32]. Figure 6(c, d) depict the CV and GCD curves of Co3O4/MCS hybrid electrode at different scan rates and current densities, respectively [33]. With the scan rate increasing, 17

ACCEPTED MANUSCRIPT the current density and integral area of the curves enhance and the specific capacitance calculated using Eq. (1) decreases, which is ascribed to the limited diffusion rate of the hydrated ions, thus causing the declining concentration on the interface between the electrode and the electrolyte [34]. And the specific capacitance calculated from GCD

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curves also decreases slightly along with increased current density, as shown in Figure 6(e), for the same reason as above-mentioned that insufficient electrochemically active materials can be supplied for the redox reactions.

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Long-term cycling stability of MCS, Co3O4 and Co3O4/MCS electrodes, acting as a major factor in the practical application of supercapacitors, was also investigated at the

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current density of 1 A g-1, and the specific capacitance of the aforementioned three electrodes after 1000 cycles is shown in Figure 6(f) [35]. The specific capacitance retention ratios for MCS, Co3O4 and Co3O4/MCS are 95.5%, 61.1% and 93.2%, respectively. The extraordinarily high cycling stability of MCS derives from the

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structural stability of carbonaceous materials and the poor cycling stability of Co3O4 mainly originates from the inevitable agglomeration of the nanoparticles and the volume variations during consecutive charge/discharge process. Noticeably, compared with the

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cycling performance of Co3O4, Co3O4/MCS shows a huge improvement in long-term

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durability ascribed to the modification of Co3O4 solving conglomeration problem, of which the reasons have been mentioned in SEM analyses above.

4. Conclusions

In summary, the ultralight and elastic Co3O4/MCS nanocomposite with remarkable

capacitive property was successfully synthesized via a facile route by taking melamine foam and ZIF-67 as precursors. ZIF-derived concave ball-in-dodecahedral Co3O4 particles were well dispersed on the conductive MCS framework through the mild stirring and simple thermolysis process. Neither pure Co3O4 nor MCS electrode exhibited better 18

ACCEPTED MANUSCRIPT specific capacitance than the Co3O4/MCS nanocomposite, which was owing to the intrinsic brilliant conductivity of MCS, extraordinarily high redox activity of Co3O4 particles with special core-shell structure and effective prevention of the nanoparticle conglomeration. Not only did the unique structure guarantee an outstanding specific

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capacitance of 1409.5 F g-1 at the current density of 0.5 A g-1, but also it supported the excellent long-term stability that 93.2% of the initial capacitance was reserved after 1000 cycles of continuous manipulation. The results may shed some light in understanding the

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thermal treatment induced morphology evolution, and also provide some hints on design

electrochemical performance.

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707-708.

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