Assembling Hollow Cobalt Sulfide Nanocages Array on Graphene-like

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Assembling Hollow Cobalt Sulfide Nanocages Array on Graphene-like Manganese Dioxide Nanosheets for Superior Electrochemical Capacitors Hao Chen, Min Qiang Wang, Yanan Yu, Heng Liu, Shi-Yu Lu, Shu-Juan Bao, and Maowen Xu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b12069 • Publication Date (Web): 18 Sep 2017 Downloaded from http://pubs.acs.org on September 18, 2017

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ACS Applied Materials & Interfaces

Assembling Hollow Cobalt Sulfide Nanocages Array

on

Graphene-like

Manganese

Dioxide

Nanosheets for Superior Electrochemical Capacitors Hao Chen, Min Qiang Wang, Yanan Yu, Heng Liu, Shi-Yu Lu, Shu-Juan Bao*, Maowen Xu Institute for Clean Energy & Advanced Materials, Southwest University, Chongqing 400715, P. R. China * E-mail: [email protected]. ABSTRACT: Metal-origanic Frameworks (MOF)-derived hollow cobalt sulfides have attracted extensive attention due to their porous shell that provides rich redox reactions for energy storage. However, their ultra-dispersed structure and large size of MOFs precursors result in relatively low conductivity, stability, and low tap density .Therefore, the construction of an array of continuous hollow cages and tailor the inner cavity of MOF-derived materials is very effective for enhancing the electrochemical performance. Herein, we in situ assembled small Co-based zeolitic imidazolate framework (ZIF-67) on the both sides of negatively charged MnO2 nanosheets to fabricate a hierarchical sandwich-type composite with hollow cobalt sulfide nanocages/graphene-like MnO2. The graphene-like MnO2 nanosheets not only acted as a structure-directing agent grow ZIF-67 array, also as a promising electroactive material of KEYWORDS: Metal-organic frameworks, Manganese dioxide nanosheets, Cobalt sulfide, Hollow nanocages array, Electrochemical capacitors

ACS Paragon Plus Environment

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Electrochemical Capacitors to provide capacitance. As an electrode material of supercapacitors, the as-prepared composites exhibit high specific capacitance (1635 F g-1 at 1 A g-1), great rate performance (reaching 1160 F g-1 at 10 A g-1), and excellent cycling stability (80% retention after 5000 cycles). The outstanding electrochemical properties of our designed materials can be attributed the unique nanostructure that improved electrical conductivity, created more reactive active sites, and increased the diffusion pathway for electrolyte ions.

1. INTRODUCTION The fast depletion of fossil fuels and environmental crises from global warming have stimulated tremendous research efforts to exploit sustainable and renewable energy sources, energy storage systems, as well as relevant advanced technologies. Electrochemical capacitors (ECs), as a type of energy storage device, have aroused widespread attention due to their fast charging/discharging feature, high power capacity, long cycle life, and excellent safety. These properties may fill the power density gap between conventional capacity and Li ion batteries, making them promising candidates to power mobile electronics, hybrid electric vehicles, and among other devices.1-4 Due to the composition and microstructure of electrode materials play a pivotal role in determining the electrochemical performance of advanced energy storage devices. The latest research and development in this area have focused on exploring novel electrode materials

for

next-generation

ECs.

Various

carbon

materials,

transition

metal

oxides/hydroxides/sulfides, and conducting polymers have been the most commonly reported three major electrode materials.5-11 However, as is well known, each material has its own obvious advantages and disadvantages. Generally, carbon-based materials, a typical electrode material, store capacitance from the reversible adsorption-desorption of ions on their surfaces,


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leading to a high power density and long cycle life but low energy density. In contrast, the capacitance of metal oxides/hydroxides/sulfides and conducting polymers is derived from a redox-type process similar to the one in batteries that produces high energy density but is kinetically unfavorable and has poor cycling stability.12

To bridge the performance gap between these materials, plentiful studies have designed various nanostructures to construct effective nanoarchitectures as a vital prerequisite for boosting electrochemical performance.13 Particularly, hollow nanostructures offer great benefits for the construction of advanced electrodes because they combine advantages of both hollow cavity and porous thin wall. Cavities in the hollow structure are beneficial for increasing specific capacitance due to the large contact area of electrolyte-materials and higher content of active species. The porous walls consist of nanosize subunits that ensure a larger specific surface area and greatly shorten the transport distance of ions, making it possible to charge and discharge more rapidly.14-16 Recently, metal-organic frameworks (MOFs), formed by metal ions and rigid organic molecules into a periodically porous architecture, have been proven to be an appealing precursor, or a sacrificial template, for the construction of hollow structures. For example, Chen's group reported hollow nano-polyhedra-layered double hydroxides using ZIF-67 as a template for electrode material of supercapacitors with a specific capacitance of 1203 F g-1 at 1 A g-1.17 Lou's group reported an onion-like hollow NiCo2S4 derived from Co-based coordination polymer spheres, which delivered a specific capacitance of 1016 A g-1 at 2 A g-1 and was used as an electrode material for supercapacitors.18 However, common drawbacks of MOF-derived hollow structures include dispersible structures and a large cavity resulting from their dispersible structure and large-sized MOF materials, which cause relatively low conductivity, stability, and tap density due to the independent particles structure and excess empty space within the


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particles. In order to enhance the specific capacitance, stability, and power densities of ECs fabricated from these hollow materials, the independent hollow particles should form a continuous conductive network and the inner cavity of the hollow structure should be confined on a nanoscale.16,19-21 More recently, direct assembly of low-dimensional electroactive materials on substrates is particularly attractive due to the formation of a 3D interconnected porous structure, which is significant for providing a sufficient porous channel, large surface area, and well-defined electrolyte diffusion pathways. Furthermore, electroactive nanostructure materials direct adhered on the substrates form a continuous conductive network that can facilitate charge transport, reducing internal resistance and boosting the cycling stability of ECs.13 For example, Fan's group reported a 3D Co3O4@MnO2 core/shell array as the electrode materials of supercapacitors, which possessed higher capacitance and maintained a better cyclic stability than pristine Co3O4.12 Yin's group prepared a 3D structured graphene wrapped MOF-derived, Codoped porous carbon, which exhibited higher specific capacitance and better cycling stability than single MOF-derived Co-doped porous carbon when used as an electrode material in a battery.22 Based on the above analysis and examples, we effectively designed and synthesized an integrated smart architecture that holds great potential to achieve superior electrochemical performance, in which the advantages of the architecture and electroactive materials of each component were fully manifested.

In our work, cobalt sulfide with rich redox reactions and manganese dioxide with high theoretical specific capacitance (1300 F g-1) were selected as the promising electroactive materials to construct an electrode of ECs.9, 23 As shown in Figure 1 (a) and (b), very slack MnO2, consisting of ultrathin layer sheets with rich folds similar to graphene, were prepared in our experiment. Interestingly, we found that the surface of the as-prepared MnO2 nanosheets


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possessed rich functional groups (Figure 1(c)) and negative charges (Figure 1(d)), which inspired use of the MnO2 nanosheets as a structure-directing agent to assemble other nano-electroactive materials on the sheets as a method to construct a 3D porous architecture. As shown in Figure. 2(a), cobalt ions were absorbed on the surface of negatively charged MnO2 ultrathin nanosheets by electrostatic interactions, providing nucleation sites for the formation of ZIF-67. After the addition of 2-methylimidazole during the hydrothermal process, MnO2 nanosheets covered by Co2+ could restrain the growth of ZIF-67. A uniform network of small ZIF-67 nanocubes continuously assembled on the surface of MnO2 nanosheets to form a sandwich-type structure (denoted as ZIF-67@MnO2). Via a simple sulfuration process, these ZIF-67 nanocubes transformed into hollow cobalt sulfides nanocages (denoted as CoSx@MnO2) with a small size (about 50 nm) and ultra-thin shell (about 10 nm) and still firmly anchored on both sides of the MnO2 nanosheets. In this approach, the negatively charged ultrathin MnO2 nanosheets acted as a structure-directing agent to synthesize the special nanostructure and also as a promising electroactive material of ECs to provide redox capacitance. As an ECs electrode material, our designed 3D CoSx@MnO2 composites delivered a high specific capacity (1635 F g-1 at 1 A g-1), outstanding rate capability (remained 71% from 1 to 10 A g-1), and excellent cycling stability (81% retention after 5000 cycles at 5 A g-1), which is far better than those of single CoSx (1013 F g-1 at 1 A g-1, remained 65% from 1 to 10 A g-1, and 45% retention after 5000 cycles at 5 A g-1), and MnO2 (229 F g-1 at 1 A g-1). The excellent electrochemical property of CoSx@MnO2 is attributed to the following reasons: (1) The hierarchical 3D porous nanostructure of our designed materials was advantageous for the fast diffusion of electrolytes. (2) Continuous hollow nanostructures formed a continuous microcosmic conductive network and greatly improved the electrical conductivity. (3) Small-sized CoSx nanocages with ultrathin shells assembled on the


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MnO2 nanosheets, offering a bigger surface area and more reactive active sites. Furthermore, a hybrid supercapacitor was also fabricated in our work, which obtained a high-energy density of 49.56 Wh kg-1 at the power density of 800 W kg-1. Our proposed synthetic methods and designed novel nanostructured hybrids offer an effective opportunity to assemble various continuous hollow-structure materials on the ultrathin negatively charged surface of MnO2 or on the surface of other negatively charged electroactive materials.

2. EXPERIMENTAL SECTION Synthesis

All chemicals were of analytical grade and were purchased from commercial sources without further purification.

Preparation of MnO2: Manganese acetate acid (0.50 g) and ethylenediaminetetraacetic acid disodium salt (EDTA, 1.5 g) were dissolved into 50 mL deionized water at 30 °C under magnetic stirring. Subsequently, 50 mL 0.25 M NaOH aqueous solution was added dropwise to the above solution. Then, 50 mL 0.12 M K2S2O8 aqueous solution was added dropwise to initiate the chemical reaction. The mixed solution was kept at 40 °C for 12 h, and then the obtained precipitates were filtered, washed with deionized water several times, and dried at 80 °C for 12 h to obtain the ultrathin MnO2 nanosheets. Preparation of ZIF-67@MnO2: 0.02g of MnO2 nanosheets was first uniformly dispersed in 10 mL methanol under ultrasonication for 2 min, and then 0.16 g Co(NO3)2·6H2O was dissolved into the above solution under stirring for 30 min at room temperature. Subsequently, 6 mL CH3OH solution containing 0.33 g 2-methylimidazole was added dropwise to the above solution.


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With continuous stirring for 2 h, the obtained precipitate was collected by centrifugation, washed with methanol several times, and then dried at 60 °C for 12 h. For comparison, ZIF-67 was also synthesized following the same procedure without adding MnO2 nanosheets. Preparation of CoSx@MnO2: The as-prepared ZIF-67@MnO2 (0.02 g) and thioacetamide (0.060 g) were dissolved into 40 mL ethanol with stirring. The resultant solution was transferred into a sealed glass bottle and heated at 80 °C for 3 h under continuous stirring. Subsequently, the products were collected by centrifugation, washed with ethanol several times, and then dried at 60 °C for 12 h. For comparison, the CoSx was also synthesized following the same procedure, where ZIF-67@MnO2 was replaced with ZIF-67.

Materials characterization. The surface morphology and structure of the as-prepared samples were characterized using scanning electron microscopy (FESEM, JSM-7800F) and transmission electron microscopy (TEM, JEOL 2100). The XRD pattern of the as-papered samples were recorded using powder X-ray diffraction (Shimadzu XRD-7000) with CuKα radiation. The composition and surface chemical states of the as-prepared samples were characterized by energy-dispersive X-ray spectroscopy (EDS, INCA X-Max 250) and X-ray photoelectron spectroscopy (XPS, Escalab 250xi). Nitrogen adsorption-desorption isotherms were recorded on Autosorb-1 (Quantachrome Instruments). Inductively coupled plasma mass spectrometry (ICPMS) analysis was performed on PerkinElmer PE Optima 8000. The specific surface area was obtained using a multipoint BET model, and the pore size distribution was obtained by the BJH model. The infrared (IR) spectra were characterized using an IR device (Nicolet Model Nexus 470), and zeta potential was obtained using the Nano ZS90 (Malvern Instruments).


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Electrochemical measurements. The electrochemical performances of the as-prepared samples were estimated in a three-electrode cell using 2 M KOH as the electrolyte. The working electrode was prepared by mixing the active material (75 wt %), acetylene black (15 wt %), and PTFE (10 wt %). The slurry coated onto the nickel foam (1×1 cm and 1 mg cm-2) and was then dried at 60 °C for 12 h. The platinum foil was used as a counter electrode and Hg/HgO electrode as the reference electrode. The hybrid supercapacitors (HSCs) were prepared using as-prepared CoSx@MnO2 as the positive electrode materials and bio-derived active carbon (BAC) 24 as the negative electrode material. The preparation method of the negative electrode was same as that for the working electrode. Electrochemical impedance spectroscopy (EIS) was recorded within 0.01 to 100000 Hz at open-circuit potential. Cyclic voltammetry (CV) and EIS were performed on an electrochemical workstation (CHI 760E Shanghai Chenhua, China). Charge and discharge measurements were performed on Land CT2001A test system (Wuhan LAND electronics Co., Ltd, China).

3. RESULTS AND DISCUSSION The microstructure and morphology of the MnO2 was shown in the Figure. 1(a) and 1(b), respectively. It is clear that the as-prepared MnO2 was very slack and consisted of ultrathin layer sheets with rich folds liked that of graphene reported in previous literature.25 TEM images further display its ultrathin two-dimensional (2D) graphene-like structure. FTIR was used to characterize the function groups of MnO2 nanosheets, and the results are shown in Figure. 1(c). The bands around 3460 and 1640 cm-1 are respective of the stretching vibrations of interlayer water molecules and bending vibrations of hydroxyl groups, respectively. The absorption peaks at 1380 and 1070 cm-1 may be assigned to the interaction of Mn with the layered inorganic host


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species, such as Na+ and H+, while the peak at 460 cm-1 can be attributed to Mn-O stretching vibrations.26 To examine the electrostatic interaction between Co2+ and MnO2, the zeta potentials of MnO2 and MnO2/Co2+ suspension were measured. As shown in Figure. 1(d), the zeta potential values of MnO2 and MnO2/Co2+ were determined to be -40.7 and +28.1 mV, respectively. It is clear that the as-prepared MnO2 displayed a markedly negatively charged surface, which may be attributed to the numerous functional groups on the surface of MnO2 and the ultra-thin layer structure. After mixing with Co2+ solution, the MnO2 surface as fully covered by Co2+ via electrostatic interaction.27-28

Figure 1 (a), (b) FESEM and TEM images of the as-prepared ultrathin MnO2 nanosheets, respectively. (c) FT-IR spectra of MnO2. (d) Zeta potential of the pristine MnO2 and MnO2/Co2+ dispersed in DI water


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Figure 2 (a) The schematic synthesis of CoSx@MnO2. (b) FESEM images of ZIF-67@MnO2. (c) TEM image of sandwich-type ZIF-67@MnO2. (d) FESEM images of ZIF-67@MnO2. (e) FESEM image of CoSx@MnO2. (f), (g) TEM images of CoSx@MnO2 with different magnification.

The in situ assembling and confined growing process of ZIF-67 on the surface of ultrathin MnO2 nanosheets, which further transferred to hollow structured CoSx and firmly anchored on the surface of MnO2, were observed by FESEM and TEM. As seen in Figure. 2(b)-(d), ZIF-67 nanocubes around 50 nm grew on both sides of MnO2 to construct a sandwich-structured


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composite, and the graphene-like ultrathin MnO2 acted as a template and binder to connect the ZIF-67 and form a continuous network. After sulfuration, the sandwich-type structure of ZIF67@MnO2 was maintained perfectly. The high-resolution FESEM image (Figure. 2(e)) of CoSx@MnO2 reveals that hierarchical network of continuous hollow nanocages uniformly formed on the surface of MnO2. More details of the as-prepared composites were further observed by TEM (Figure. 2(f)), which reveals the uniform distribution of CoSx hollow nanocages on the graphene-like ultrathin MnO2 to form a continuous hollow CoSx nanocage network. The inner cavity can be observed by contrast the hollow interiors and shells, which were constructed of compactly arranged nanoparticles with a thickness of around 10 nm (Figure. 2(g)). The ultra-thin shell thickness can well facilitate the diffusion of the electrolyte throughout the whole structure and significantly enhance the rate capability of supercapacitors. To investigate the role of MnO2 in the formation of the sandwich-type composite structure, pure ZIF-67 crystals were also synthesized and sulphureted under the same conditions. As shown in Figures S2a and 2b, the ZIF-67 particles were dispersible, and the size of ZIF-67 particles were around 500 nm, which is far bigger than those grown on the surface of MnO2. To further investigate the microstructure of the as-prepared samples, N2 adsorption-desorption isotherms and corresponding pore size distribution of CoSx@MnO2, CoSx and MnO2 were obtained, as shown in Figure. S3a and S3b. The II-type adsorption-desorption isotherm and H3type hysteresis loops indicate the mesoporous characterization of the three as-prepared samples. The surface area of the hierarchical CoSx@MnO2 nanostructures was calculated to be 100.5 m2 g-1 using the multi-point BET method, which is greater than that of the CoSx hollow cages (42.1 m2 g-1) and ultrathin MnO2 nanosheets (80.0 m2 g-1). The higher specific surface area of the


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CoSx@MnO2 should ascribe to small size (about 50 nm) and ultra-thin shell (about 10 nm) of the CoSx nanocages array. The crystal structures of the as-prepared products were characterized by XRD. As seen in Figure. 3(a), the XRD patterns of ultrathin MnO2 nanosheets were well indexed to delta-phase MnO2 (JCPDS card no. 43-1456) without any impurity peaks. Furthermore, from the Figure S4, the XRD pattern of ZIF-67@MnO2 was well indexed to delta-phase MnO2 and simulated ZIF-67. The XRD patterns of the CoSx and CoSx@MnO2 show a broad peak around 23º, indicating the amorphous nature of these samples, which is similar to those previously reported.29 However, no MnO2 peaks were observed in the XRD patterns of CoSx@MnO2 sample, which may be accountable to the following reasons: (1) the surface of MnO2 was covered by amorphous CoSx; (2) during the sulfuration process, a portion of surface MnO2 was sulfureted to form amorphous manganese sulfide, so that the diffraction peaks of MnO2 were flooded by amorphous peaks. The EDS were further used to analyze the elemental compositions and distribution of the CoSx@MnO2 (Figure. S7a) and CoSx (Figure. S7b). As displayed in Figure. S4a, the atomic ratio was around 1:1.28 for Co:S, which is similar to that of CoSx (around 1:1.30 for Co:S, Figure. S4b), and the atomic ratio of Co:Mn in CoSX@MnO2 was approximately 3:1. The EDS elemental mapping analysis also suggests that the various elements were homogeneously distributed throughout the whole sample (Inset of Figure. S7a and S7b). The atomic ratio of Co and Mn in CoSX@MnO2 was also measured by ICP (Co/Mn = 2.85:1), which is very similar to the value (3:1) obtained from EDS analysis.


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Figure 3 (a) XRD patterns (b) High resolution S 2p XPS spectra of CoSX@MnO2. (c) High resolution Co 2p XPS spectra of CoSx@MnO2 and CoSx. (d) High resolution Mn 2p XPS spectra of CoSx@MnO2 and MnO2.

XPS was used to characterize the chemical states of elements on the surface of MnO2, CoSx, and CoSx@MnO2. The S 2p spectrum of CoSx@MnO2 is shown in Figure. 2(b) and displays two main peaks assigned to S2- (162.1 and 162.2 eV) and SOx (168.6 and 170.0 eV). The SOx peaks may have been caused by adsorbed oxygen on the active surface of CoSx@MnO2. The spectrum of Co 2p in CoSx@MnO2 (Figure. 3(c)) may be best described by two spin-orbit doublets and two satellite peaks (identified as “Sat.”). The first doublets at 779.0 and 794.0 eV and the second


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doublets at 781.3 and 797.4 eV are ascribed to Co3+ and Co2+, respectively.30 The existence of Co3+ indicates that some Co2+ on the surface of sample was oxidized. Furthermore as shown in Figure. 3(e), the binding energies and intensity of Co 2p in CoSx@MnO2 are very similar to CoSx, which indicating the similar chemical state of Co on the surface of CoSx@MnO2 and CoSx. The Mn 2p spectra of MnO2 decomposed into four peaks (Figure. 3(d)), where the two peaks around ~642.0 and ~652.9 eV are ascribed to Mn 2p3/2 and Mn 2p1/2 of the Mn3+ species, and the two peaks around ~642.7 and ~654.2 eV re ascribed to the Mn4+.31 It should be noted that the binding energies of Mn3+ 2p in CoSX@MnO2 were negatively shifted compared to those of Mn3+ 2p in MnO2, and the intensity of Mn3+ 2p in CoSx@MnO2 was stronger than Mn3+ 2p in MnO2. The latter result is good proof that a portion of Mn4+ on the surface of MnO2 was reduced to Mn3+ and formed amorphous sulfide during the sulfuration process. This is consistent with the XRD result that do not show any diffraction peaks of MnO2 in the CoSx@MnO2. The electrochemical performance of as-prepared samples as the active electrode material of supercapacitors was first measured by CV. It is apparent that all CV curves show a couple of redox peaks within the measured potential range. For CoSx, a pair of oxidation/reduction peaks around 0.31 and 0.57 V originated from the faradaic redox reaction related to Co2+/Co3+. A couple of redox peaks appeared around 0.34 and 0.46 V for ultrathin MnO2 nanosheets, which are ascribed to the insertion/extraction of K+ on the ultrathin MnO2 nanosheets.12,

25

After

combining the above two components, the resulting CoSx@MnO2 displayed redox peaks at about 0.26 and 0.54 V, which are attributed to the synergistic redox contribution from the redox reaction of CoSx nanocages and partially vulcanized MnO2.Compared to hollow CoSx hollow cages and ultrathin MnO2 nanosheets, the peak current and area of the CV curve of CoSx@MnO2 are obviously larger, indicating that a larger capacitance of CoSx@MnO2 was obtained. The area


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and peakcurrent of the CV curve of the nickel foam was very small, which suggests that the capacitance of Ni foam is negligible compared to the electrode materials.

Figure 4 (a) CV curves at a scan rate of 50 mv s-1; (b) Nyquist plots; (c) Galvanostatic charge/discharge curves at 0.5 A g-1; (d) specific capacitance of the as-prepared samples at different current densities; and (e) Cyclic stability of sandwich-structured CoSx@MnO2 at 5 A g1

.


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The ion and charge transfer characteristics of the as-prepared samples were investigated using EIS measurement. The corresponding Nyquist plot (Figure. 4(b)) was obtained at open circuit potential. Generally, the diameter of the semicircle at the high frequency region (inset of Figure. 4(b)) represents the charge transfer resistance, and the straight line in the low-frequency region (Figure. 4(b)) refers to the diffusive resistance of the electrolyte in the electrode and host materials. It can be determined from Figure. 4(b), CoSx@MnO2 had the smallest electrochemical reaction resistance and ion diffusion resistance among the three samples, which is attributed to its hierarchical 3D continuous hollow nanostructured network. It is well accepted that galvanostatic charge-discharge examination can be used to estimate the specific capacitance of electrode materials. As shown in Figure. 4(c), two different steps appeared during the charge and discharge of all samples. The parallel to vertical direction of charge and discharge curves on the first steps indicates double-layer capacitance and pseudocapacitance behavior, whereas the parallel to horizontal direction of the potential vs. time (0.5~0.3 V) reveals battery-like behavior.4 The capacitance of as-prepared CoSX@MnO2, CoSx cages, and MnO2 nanosheets were calculated to be 1720, 1013, and 230 F g-1, respectively, at 0.5 A g-1. Obviously, hierarchical 3D sandwich-type continuous hollow CoSx@MnO2 possessed the highest specific capacitance, which is in good agreement with observations based on the CV curves. The rate behavior of hierarchical sandwich-type CoSX@MnO2 can be seen in Figure. S8b, and the analogous shape of the galvanostatic charge/discharge curves at different current densities is suggestive of good charge-transport behaviors in the hierarchical 3D sheet-like, continuous hollow-structure CoSx@MnO2 electrode. Figure. 4(d) illustrates the current density dependence on the specific capacitance of the three as-prepared samples. Within the current density test range, the CoSx@MnO2 always delivered a


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higher specific capacitance than the other samples. Moreover, after increasing the current density from 1 to 10 A g-1, the specific capacitance of CoSx@MnO2 still was maintained up to 71%, while hollow CoSx was only maintained at 65%. Durability is an important parameter for the practical application of electrode materials. The cycling stability of CoSx@MnO2 and hollow CoSx electrode was evaluated based on the charging/discharging cycling measurements at 5 A g-1 for 5000 cycles, as presented in Figure. 4(e). Only 20% capacitance loss was observed for CoSx@MnO2, which is more stable than that of hollow CoSx with 46% capacitance retention.

Figure 5 (a) Charge-storage mechanism of the CoSx@MnO2 cathode and the activated carbon anode in the hybrid supercapacitors system. (b) CV curves at different scan rates and (c) charging-discharging curves at different current densities of the hybrid supercapacitors. (d) Cyclic stability of the hybrid system at 5 A g-1. (e) Ragone plot of the CoSx@MnO2//AC hybrid supercapacitors, and related reported elsewhere. (f) Digital image of the hybrid supercapacitors cell voltage and the inset of a red-light-emitting diode lighted by the hybrid supercapacitor


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device.

The above results indicate that the hierarchical sandwich-type CoSx@MnO2 is a highly promising candidate for SCs. Nevertheless, the energy density of SCs is inferior to that of batteries, which limits their widespread application in the future. Hence, it is imperative to boost the energy density of SCs without sacrificing their power capability and cycling ability to fulfill the growing demand for energy. A promising approach to address these issues is to fabricate hybrid supercapacitors (HSCs) that combine the advantages of both batteries and SCs and can deliver high energy density and power capability simultaneously. Hence, a HSC device was fabricated in our work by employing CoSx@MnO2 as the positive electrode material and bioderived active carbon (BAC) as the negative electrode. According to measurements and calculations of the three-electrode (Figure. S9), the specific capacitance of BAC was determined to be 250 F g-1 at 1 A g-1 with 1.0 V potential window, and the optimized mass ratio between the negative and positive materials was around 0.15.32 Figure. 5(b) shows the CV curves of the CoSx@MnO2//BAC HSC device at different scan rates, where a potential window of 1.6 V was operated without significant polarization. The galvanostatic charge-discharge curve (Figure. 5(c)) of the CoSx@MnO2//BAC device shows good symmetry and a linear slope, demonstrating good capacitive behavior. Meanwhile, the CoSx@MnO2//BAC device delivered a high specific capacitance (145 F g-1) at 1A g-1. Figure. 5(d) exhibits the excellent cycling stability of the CoSx@MnO2//BAC device with its capacitance retention reaching 92% even after 5000 cycles. The power and energy properties, as critical evaluating parameters of HSCs, were calculated and used in a Ragone plot (Figure. 5(e)). The CoSx@MnO2//BAC hybrid supercapacitors exhibited an energy density of 49.56 Wh kg-1 at a power density of 800 W kg-1, and a power density of


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8000 W kg-1 at an energy density of 33.75 Wh kg-1, which is superior to many previously reported HSCs.10, 29, 33-36

4. CONCLUSIONS In summary, a 3D hierarchical sandwich-typed hollow CoSx@MnO2 composite has been successfully fabricated using ZIF-67 as a sacrifice template. We discovered that the ultrathin MnO2 nanosheets with a positively charged surface not only a promising electroactive material that provides redox capacitance in ECs and also acts as a structure-directing agent to design special nanostructures.

The

as-prepared

CoSx@MnO2

as

the

electrode

material

for

supercapacitors delivered a desirably high specific capacitance (1720 F g-1 at 0.5 A g-1) with excellent rate performance and cycling stability (maintained up to 71% from 1 to 10 A g-1 and only 20% capacitance loss at 5 A g-1 for 5000 cycles). These advantages render CoSx@MnO2 attractive for practical applications in hybrid supercapacitors, and the as-assembled CoSx@MnO2//BAC hybrid supercapacitor shows higher energy density and power density than many previously reported HSCs. The excellent electrochemical properties of CoSx@MnO2 are attributed to several structural properties, which are described as follows: (1) The hierarchical 3D porous structure reduces the diffusion path length of electrolyte ions. (2) The continuous hollow nanocages form a continuous microcosmic conductive network, which greatly improves the electrical conductivity of the materials. (3) The small cavity and ultrathin shell offer a large surface area and more active sites. Thus, the as-prepared novel 3D sandwich-type continuous hollow nanostructure materials are promising for energy storage, while our proposed method could be a universal “in situ assembling” approach to synthesize various continuous hollow structure materials on the ultrathin negatively charged surface of MnO2 or the surface of other negatively charged electroactive materials.


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ASSOCIATED CONTENT

Supporting Information FESEM image of sandwich-type ZIF-67@MnO2, FESEM image of hierarchical sandwich-type CoSx@MnO2, FESEM images of ZIF-67, FESEM images of dispersible hollow CoSx nanocages, N2 adsorption/desorption isotherms of dispersible hollow CoSx nanocages, ultrathin MnO2 Nanosheets and hierarchical sandwich-type CoSx@MnO2, corresponding pore-size distribution, XRD pattern of the ZIF-67 and sandwich-type ZIF-67@MnO2, The FESEM images of pristine MnO2, sulfuretted MnO2 prepared with the same sulfuration process of ZIF67@MnO2, sulfuretted MnO2 prepared with the same sulfuration process of ZIF67@MnO2 expect double TAA concentration, The XRD pattern of pristine MnO2, sulfuretted MnO2 prepared with the same sulfuration process of ZIF67@MnO2, sulfuretted MnO2 prepared with the same sulfuration process of ZIF67@MnO2 expect double TAA concentration, EDS element composition diagram and inset is the corresponding elemental mapping images of hierarchical sandwich-type CoSx@MnO2 and dispersible hollow CoSx nanocages, Elemental content analysis (by EDS) of dispersible hollow CoSx nanocages and hierarchical sandwich-type CoSx@MnO2, CV curves of hierarchical sandwich-type CoSx@MnO2 at different scan rate, Galvanostatic charge/discharge curves of hierarchical sandwich-type CoSx@MnO2 at different current density, CV curves and charge-discharging curves of bio-derived active carbon, Specific capacitance at different current density of CoSx@MnO2//AC device. This material is available free of charge via the Internet at http://pubs.acs.org.


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AUTHOR INFORMATION

Corresponding Author * E-mail: [email protected]. Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS

This work is financially supported by the National Natural Science Foundation of China (No.

21773188),

Fundamental

Research

Funds

for

the

Central

Universities

(XDJK2017D003, XDJK2017B055), Program for Excellent Talents in Chongqing (102060-20600218), and Program for Innovation Team Building at Institutions of Higher Education in Chongqing (CXTDX201601011) and Chongqing Key Laboratory for Advanced Materials and Technologies.

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2014, 8, 9531-9541.


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Table of Contents Graphic


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Assembling Hollow Cobalt Sulfide Nanocages Array on Graphene-like

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